Loss of sharpness

[corinnea]

Hi All,

I know this may sound like an incredibly daft question and I've searched endlessly to try and find the answer on google to come up with not a great deal of information that makes sense to me (was never very gifted with the maths side of things) and thought I'd be brave and ask on her.

I have a 24-70mm sigma DX lens and am trying to work out the distance I can shoot (ie how many metres from my subject/s) before I lose the sharpness and detail in the image , is there a simple way to work out the distance for each different lens that you use?

Appreciate the comments.

Cheers,
CA

[herbert]

Hi CA,

Welcome to the forum.

Image sharpness will be effected by the image sensor, the lens and the distance to the subject. It can be measured using various metrics. For example the number of line pairs that can be distinguished per unit distance. Think of this as drawing thin white and black lines closer and closer together. Eventually you will not be able to see individual lines. They will look grey.

This sort of sharpness measure is done at the optimum (usually the closest) distance that the lens can focus. This is because the sharpness would just drop off as the subject was further away (just like it does with your own eyes).

I have not seen a tool where you can enter your image sensor size, the lens and the distance to the subject and get out the sharpness. However image sharpness is subjective. You may be able to see all the details you want for one subject and at the same distance not be able to see any details for another.

Your best solution is to take a lot of pictures of typical subjects at various distances. You can then judge the distance which is sharp enough for you. This sort of process is just a way of learning how your lens performs.

Hope this helps.

Alex

[Snarkbyte]

Hi CA, and welcome to CiC.

Please pardon me if I have the wrong impression, but we see a lot of photography newbies here, and terminology can be a rather confusing. (If you are a newbie to photography, you've come to the right place... CiC is a great place to learn!)

I just have the impression that what you're really asking about is depth of field, which is how much of the foreground and background is in focus. If this is the case, I recommend reading the tutorial on depth of field ( https://www.cambridgeincolour.com/tu...h-of-field.htm ), which will help you understand how to control it. The tutorial also includes a DOF calculator. In actual practice, the calculation isn't all that useful... most people just use the DOF preview button on the camera (most DSLRs have one), and to some extent, rely on experience.

I apologize if this is completely off-topic, but the question of sharpness vs. distance just made me think....

[Photon Hacker]

Hello.

I suppose you can't find an answer because the question is bad formulated. There isn't a dimension named "sharpness" or "detail", those are subjective concepts and depend on the image you're talking about and its subject. Think of the difference between a sharp portrait and a sharp image of the moon. The difference in size of the detail you aim to resolve is very large, so is the distance of your subject.

[Geoff F]

There is a bit of information about the specs of this lens at this review site http://www.photozone.de/canon-eos/31...report--review

I see from that information your minimum focus distance is 0.4 m. Do you want to know the minimum focusing distance at other lens lengths, or do you want to know the area which is actually in sharp focus at a number of distances and apertures, ie Depth of Field, as Al mentioned?

Some of the old film lenses had markings on the lens barrel to indicate this but I never found them to be totally accurate.

I seem to remember some formula for calculating DOF at various apertures but have never used this myself.

If you are talking about just the minimum distance between you and your subject, auto focus should fail to lock on if you get too close.

One other point to mention is that all lenses have a preferred aperture range, sometimes called the 'sweet spot' where they will be at their sharpest. Usually around the middle of the range, say F8 for example.

Incidentally, when I first started with photography, several years ago, I wrongly assumed that smaller apertures produced the best quality; ie F16 would be sharper than F8. It took me quite a lot of frustrating experimentation before eventually reading about the 'sweet spot'.

ps. And one other thing to consider about problems with sharpness at longer lengths is the 'quality of the air'. The amount of moisture in the air will cause variations in 'clarity' and the greater the distance the more distortion will be caused.

[corinnea]

Thank you all so much for your responses - I apologise for any confusion in my question as I do believe I have not delivered it very well - I think what I am trying to establish is the maximum distance my lens can shoot (or any lens for that matter), I took a series of photos of my daughter the other day who was on the other side of the backyard which was at least 5 metres away and I had my lens at it's longest focal length which is 70mm and whilst the images on the LCD look sharp, when they are cropped for instance they are not sharp at all, so I am trying to establish how far from my subject can I be.

I appreciate all your comments and taking the time to make them!

[epmi314]

Thank you all so much for your responses - I apologise for any confusion in my question as I do believe I have not delivered it very well - I think what I am trying to establish is the maximum distance my lens can shoot (or any lens for that matter), I took a series of photos of my daughter the other day who was on the other side of the backyard which was at least 5 metres away and I had my lens at it's longest focal length which is 70mm and whilst the images on the LCD look sharp, when they are cropped for instance they are not sharp at all, so I am trying to establish how far from my subject can I be.

I appreciate all your comments and taking the time to make them!

I don't believe that lens has Sigma's OS system which stablizes the image. The "sharpness" you may be lacking could also have to do with camera shake depending on what shutter speed you are working with.

A really good idea would be to post a couple of examples with the camera settings. It may allow some of the folks on this site to identify what you could do to improve your results.

You should be able to get a really sharp image with that lens with the correct set up.

[Andrew1]

Your question is far from daft. Many of us (read "all") have at one time or another questioned our own results whether it be from technical or human causes.

Without getting into the obvious cost/quality discussion, different lenses have different characteristics and on zooms that difference can manifest itself even between lenses of the same range. That's usually not something most of us can see with our naked eye though but there are lots of cases of rear focusing. A problem with mass-produced lenses not being given the attention at the factory. My Nikon camera has an adjustment for lenses with exactly that problem. I think you need to just play with your lens a lot and get used to what you can do with it in your own hands. I shake like hell and cannot get very good shots so have used a monopod for 20 years. Set up your kit on a tripod and do your analysis of what the camera and lens can do in ideal conditions. A shutter release would help too. When you get a good idea of its performance take it off the tripod and do the same thing in-hand. How much you blow it up and the screen you're looking at it on also has an effect. To judge the results you really need to base it on a print from a trusted shop.

It just takes practice and what we get as well as what we're satisfied with differs with all of us.

[Shadowman]

Very good series of inquiries.

1. Consider lens capabilities at specific aperture, how important background is to the image (DOF), , sharpness/focus of overall image, importance of focus for a portrait.

[herbert]

I took a series of photos of my daughter the other day who was on the other side of the backyard which was at least 5 metres away and I had my lens at it's longest focal length which is 70mm and whilst the images on the LCD look sharp, when they are cropped for instance they are not sharp at all, so I am trying to establish how far from my subject can be.

Hi corrinea,

Thanks for providing more details on your shooting. You have pointed out one issue we all face. When enlarging an image you can see more detail. Doubling an image size requires a doubling of your sharpness for it to look the same quality. Your camera LCD is probably 3 inches wide. Images that look sharp at this size may not look so good when enlarged.

You may not have a requirement to print images 24x16 inches but if you crop an image to 1/4 the size and print at 6x4 then you are doing the same thing. Your technique and equipment will be under more pressure to perform.

Here are some tips:

When shooting moving people try and maintain a shutter speed of 1/125 or maybe faster for small children who never stop moving. Upping the ISO to keep the image at this speed will improve sharpness more than the loss of quality from higher ISO.

For moving subjects use the continuous focus mode on your camera.

Shoot a lot more pictures by using the burst mode on your camera to take multiple images. One of them may be better than the others. You can delete the out of focus ones.

Practice your camera holding technique. You need to brace the camera to stop it shaking. Pushing it against your face helps but it may be useful to lean against a wall or rest on something like a monopod.

You can see how shaky you are by shooting some text at 5m distance in continuous burst mode. Zoom your image to maximum magnification and move between the images. See how many pixels the consecutive images are shifted by. Don't be alarmed. Everyone shakes. This is why we use high shutter speeds. Ideally what you are looking for is each consecutive image to be sharp even if they are shifted relative to each other. You need to work out for your 5m distance the shutter speed at which each image is sharp. This may be 1/125, it may be 1/30 or 1/500. Using this speed with your new practiced technique will give you the best starting point for photos of your daughter. Then you just need to time the shot right so she is not moving too much.

Good luck. Post back some images on the 'People & Pets' forum and people will be happy to provide tips on how to improve the images if you ask.

Alex

[corinnea]

Thanks Alex - I am wondering if the reason also that I am losing sharpness is that I am half depressing the shutter then focusing and recomposing?? with the particular photos I am concerned about the background is sharp yet the two girls aren't (well not as much as I would like anyway), but in saying that I looked at a few of the images that I took and they are not all like that, which brings me back to wondering if it's because I am recomposing - also do you have any tips when your subjects are on different focal lengths where to best focus - I constantly seem to have difficulty in knowing where to focus when I have my daughter and her friends running around (I tend to when I am in this instance shoot with F8 or up).

Thanks so much

[Donald]

Hello there.

Like was said at the beginning of the thread, when we're setting out on our learning, it's sometimes difficult to know what the right questions are, never mind what the right answers are.

I think there are a number of issues/points involved in what you are seeking to answer:

• depth-of-field
• hyperfocal distance (which is part of the issue of depth-of-field)
• quality of the lens

are three of them.

And I think you need to just take a moment to make sure you understand each of them. It's a bit like sorting out a jumbled up ball of string - You can't find the ends until you unravel all the confused mess of tangled bits.

On your last point about focusing and recomposing. That shouldn't be the problem so long as you're focusing where you want to focus and then holding the shutter to lock the focus whilst you recompose.

You also wrote about the problem of keeping everyone in focus when they are on different focal planes; i.e. one nearer, one further away. That's all about depth of field. At f8 on a 24-70 lens you should have a pretty big depth-of field. So unless the two people are quite far apart, then they should both be in focus. But, you've got to remember - you can only ask the camera and lens to do what they are capable of doing. If you have one person close and one far away, even the best equipment in the world may not do what you want.

But if we go back to your original question:

I have a 24-70mm sigma DX lens and am trying to work out the distance I can shoot (ie how many metres from my subject/s) before I lose the sharpness and detail in the image , is there a simple way to work out the distance for each different lens that you use?

The only thing you need to think about is minimum focusing distance. There is a point at which the lens can focus no closer. That's the first thing.

There is no maximum distance. As others have already said, that's when it all comes down to the quality of the lens, the clarity of the weather conditions at the time, your ability to hold the camera rock steady (that's when you need a tripod), the size at which you're going to view the finished image ....... and at least a basic understanding of hyperfocal distance. But hyperfocal distance is something that usually comes more into play for landscape photographers rather than for those shooting people in a garden.

The hyperfocal distance at f8 at 70mm is just over 30.5 metres. That is where you would need to focus in order to get the maximum possible depth-of-field. So (as we know) everything from half that distance (15.25m) to infinity, is going to be acceptably in focus. So, your question about having the foreground and the background in focus all comes down to the question of how close is the foreground and how far away is the background.

I hope that helps to shed some light on what are a complex set of questions. So, rather than your original question being 'incredibly daft', it is in fact quite a complex question to which the answer is, I suggest, that you need to think through all the 'bits' of the question and unravel them ... just like that tangled ball of string.

[corinnea]

Thank you so so much Donald - would you mind if I posted the photo in question and another photo taken in the same shoot?

Thanks

[herbert]

Hi Corinnea,

Focus and recompose works best for static subjects. This is not the case for your children.

You could try setting the camera to continuous focus mode. Not only does this constantly update the focus but it is smart enough to work out the direction the object is moving and smoothly move the focus that way. This means it can keep the subject in focus by predicting where it will go when the autofocus cannot function, i.e. when the mirror is up and the shutter is open taking a picture. This works best on larger subjects with smooth movements. It should work better on your children than a tiny flying bird.

If you are recomposing to position the children on one side of the image you may have more success by using the focus points on that side of your camera. This will prevent you having to recompose. In the short time you do this your children may move.

Also be aware of how the autofocus operates. It will favour two things:

1. High contrast detail, e.g. a black and white picket fence
2. Closer subjects

Both of these provide a higher response to the camera auto focus. So the camera picks them when it has many options. If you are using multiple focus points on your camera then it is possible that one may be picked that is not on your children. Try switching to using a single focus point. Also note that even though the camera has a little dot for the focus point it actually covers as much as 1/3 of the frame from top to bottom. So be aware of high contrast edges close to your children. They may be picked for focus instead.

I have found that when I use the same button for focus and shutter control that it is hard to capture fast moving subject and still compose the shot. On my camera I can use a different button for the focus and turn this off on the shutter button. I can then focus when I want (i.e. the subject, or me, is moving) and leave the focus alone when the subject is still. This is often called back-button focusing because the second focus button is on the back on the camera. Let us know your camera model and we will be able to figure out what focus options you have.

Read through the points made by Donald. The depth of field is the key here. There is a lot to learn at the start but over time it gets easier. There are a lot of good tutorials on this site that help you understand focus and depth of field.

Note that if you do see some figures for your depth of field then note that they apply to an image with no camera shake or subject movement. If you are shaking then they are largely irrelevant as all the image will be blurred. This is why I suggest you keep your shutter speed high. Also note that depth of field is based on a 'standard' measure of the detail the eye can see at a set viewing distance. If you get closer you will be able to see less sharpness. This is the same as saying you should always consider the final output destination when measuring sharpness. Unfortunately we used to mainly output 6x4 or 7x5 prints when we shot film 15 years ago. Now a lot of people use a 20 inch monitor which is twice the size and they view from the same distance. This can make the pictures look worse than they are. Step back to 1 meter and see how they look.

Keep taking photos. You will get the results you want with practice.

Alex

[corinnea]

I see from that information your minimum focus distance is 0.4 m. Do you want to know the minimum focusing distance at other lens lengths, or do you want to know the area which is actually in sharp focus at a number of distances and apertures, ie Depth of Field, as Al mentioned?

Hi Geoff - do you have any pages you would suggest that go into detail about your above comment about sharp focus at a number of distances and apertures

Thanks


Some of the old film lenses had markings on the lens barrel to indicate this but I never found them to be totally accurate.

I seem to remember some formula for calculating DOF at various apertures but have never used this myself.

If you are talking about just the minimum distance between you and your subject, auto focus should fail to lock on if you get too close.

One other point to mention is that all lenses have a preferred aperture range, sometimes called the 'sweet spot' where they will be at their sharpest. Usually around the middle of the range, say F8 for example.

Incidentally, when I first started with photography, several years ago, I wrongly assumed that smaller apertures produced the best quality; ie F16 would be sharper than F8. It took me quite a lot of frustrating experimentation before eventually reading about the 'sweet spot'.

ps. And one other thing to consider about problems with sharpness at longer lengths is the 'quality of the air'. The amount of moisture in the air will cause variations in 'clarity' and the greater the distance the more distortion will be caused.[/QUOTE]

[corinnea]

Thanks Alex, I shoot with my center focus point and am trying really hard to learn to manually move my focus point instead of recomposing but it can be tricky at times

I shoot in al servo mode when im shooting the kids but only on one shot on a canon 5d mark II - i haven't read in the manual anything about back button focusing (but I could have completely missed it).

With your suggestion of the 5m text shooting, should I try it at different apertures aswell as shutter speeds?

I really feel like I am coming across as somewhat ignorant, so I appreciate all your tips and advice!!

[Donald]

Thank you so so much Donald - would you mind if I posted the photo in question and another photo taken in the same shoot?

Thanks

That would be the best thing to do. Then we can see what we're all talking about.

[Dave Humphries]

I agree we should see an image (or 3) Corinnea,

I suspect we're going to see two 5m distant kids, which, if shot on a full frame camera at 70mm, they will be quite small and I wonder if the sharpness is not being adversely affected by 'over cropping' the resulting image in order to extract the required picture form a much wider view?

Other than that, you'll learn a lot by just keep practising, using different methods.

Don't worry about coming across as 'somewhat ignorant', we all started there once.

The thing is to put in some effort to understand what's happening; either by reading the manual (and ask us more questions), or getting up to speed on a few basics from the tutorials here, if that's not what works for you, post images, absorb the feedback and post again having tried what is suggested - there's nothing we like more than see people come on in leaps and bounds

.. when I have my daughter and her friends running around ..

You may need to pan (i.e. follow) the girls with the camera, not 'recompose' against the static background if they are moving.

Welcome to the CiC forums from ...

[herbert]

With your suggestion of the 5m text shooting, should I try it at different apertures aswell as shutter speeds?

The different apertures will not make much difference. In fact you will have to change the aperture to keep the exposure constant as you change your shutter speed (if you go beyond the range of ISO changes that your camera allows).

The purpose of the test is to show you how much you are shaking when shooting a static subject. I suggested to focus on text since it is usually on a flat surface (no surrounding distractions) with good contrast. This makes it a nice subject for the autofocus. Make sure that the target is well lit. Preferably do this outside during the day. There is far more light even on a cloudy day than you have inside your house at night. This will help your autofocus fix on the text and ensure you only see blur due to your camera shake.

At fast shutter speeds, e.g. 1/4000, you should be able to see how much you have moved between shots. At lower speeds you will see that this movement is captured in the actual picture. So as the speed slows down the image will get blurred. At a speed of 1 or 2 seconds it will be very blurred. Only superman can hold a camera still for that long. The test is to determine how close to superman you are. It also provides you with the ability to try different holding techniques on your camera to see if they help.

Your Canon 5D should allow you to try back-button focus by setting it in the custom menus of the camera. See this article:

http://www.learn.usa.canon.com/resou..._article.shtml

However note that it can take a while to get used to it. It feels wrong to start with. But it might be worth a try.

I really feel like I am coming across as somewhat ignorant, so I appreciate all your tips and advice!!

Don't worry. As Dave has pointed out we all start at the beginning.

The more I know, the more I realise I don't know.

Alex

[Colin Southern]

Hi Corinnea,

In summary, what I'd be doing is ...

- Keeping the camera in 1-Shot AF mode

- Keeping your shutterspeed at least twice your focal length (eg if you're shooting at 80mm, adjust your aperture and ISO to keep it at least 1/160th) (the higher the better)

- Ensure you have an AF point over the bit you want in focus (just use the one -- don't let the camera choose from any of them)

- Make sure you're applying the correct amount of sharpening when you post process the shots (this has the potential to make the biggest difference)

A sample image to evaluate would be helpful.

Hope this helps.