Canon G2 and focal length

[oldgreygary]

Hello,

My question is specifically related to the Canon G2. It is a fixed lens camera which has a lens with focal length ranging from 7mm-21mm. 35mm equivalent is 34mm-102mm. In low light conditions I had been using f/2.8 , 1/500, ISO 100 and 7mm focal length. Unfortunately, there is no way on the camera to set a specific focal length.

Specifically, I had been testing out taking images of moving trains. I had noticed a consistency in the results in that although the images were reasonably sharp things like lettering/graphics were quite soft. By accident I left the camera at it's longest focal length of 21mm. The images from this were a lot sharper and clearer including the lettering/graphics.

I didn't quite understand why this was. In a sense using f/2.8 at the low end of focal length (7mm) seems to be a bit counter intuitive. Fstop suggests shallow depth of field but focal length suggests wide depth of field? Whereas at the longer focal length a shallower depth of field would suggest more compatibility?

Anyway, perhaps someone would like to offer some thoughts which might clear up my confusion. I am more than happy to be corrected on my understanding of this. Thanks.

Gary

[John 2]

Gary, it could be a couple of things. First of all the way it works is that the wider the aperture or the longer the focal length the shallower the depth of field. However with the sensor size of the G2, depth of field is almost a given at the distances that you will be shooting trains at and I doubt whether this is a factor. Most zoom lenses change their performance with focal length and it could be just that. More likely though is that your wide angle shots will need more enlargement on your computer screen in order to examine them than your telephoto shots. Its a fact of photographic life I'm afraid that the more you enlarge an images the more the inherent noise or even pixilation of the image will become apparent. An enlarged image can therefore, be degraded in quality. Hope this helps and if anyone thinks I've got this wrong - speak up.

[jcuknz]

I have read and heard that f/8 gives the result of between f/22 and f/32 on a SLR so working back that suggests to me that f/2.8 equals f/8 to f/11 from which one would expect quite a bit of depth of field. Have you visited the CiC tools section and played with the DoF calculator? f/2.8 only gives you limited DoF at longer focal lengths

You have a 1/1.8 sensor according to dpreview
if focused at one metre you have no DoF at full zoom .... wide end 0.73m to 1.6m
At 200m you have 135.4m from 153m to 288m ... using the wide end you have from 2.61m to infinity
I would have expected more DoF at a metre but that is what the CiC calculator tells me.

Take a photo of a ruler angled away from you at about 20 inches and focus at the 6 inch mark ... you should see a limited DoF 0.4< 0.6m

[oldgreygary]

Thanks for your replies. I did some further investigation and did 8 test shots. 4 at 7mm and 4 at 21mm. Using f/2.8, f/4, f/5.6 and f/8. I used 1/500, ISO 100. Handheld. Overcast conditions. This link shows the results. I focused on the box with the graffiti on the left hand side. Unfortunately, I don't know the distance to the box. Here are the results:

This link //http://garypocklington.smugmug.com/Other/7mmTo21mm/30030435_xnCJr3.

I suspect that if I repeated on a sunny day the 7mm shots would be a lot better?

Cheers for now

Gary

[revi]

I have read and heard that f/8 gives the result of between f/22 and f/32 on a SLR so working back that suggests to me that f/2.8 equals f/8 to f/11 from which one would expect quite a bit of depth of field. Have you visited the CiC tools section and played with the DoF calculator? f/2.8 only gives you limited DoF at longer focal lengths

You have a 1/1.8 sensor according to dpreview
if focused at one metre you have no DoF at full zoom .... wide end 0.73m to 1.6m
At 200m you have 135.4m from 153m to 288m ... using the wide end you have from 2.61m to infinity
I would have expected more DoF at a metre but that is what the CiC calculator tells me.
(...)

The CiC DoF calculator states that you have to use the actual focal length, not the "35 mm equivalent".
If I do that, 21mm setting, F2.8 @ 200 m gives me a DoF of 30m to infinity...

[Dave Humphries]

Hi Gary,

Having looked at your shots, I would suggest that John's suggestion in post #2 is probably the largest factor at play here.

You have so far only tested at the two extremes (7mm and 21mm), I wonder whether it would be a little sharper at 10mm? However, enlargement when comparing 10mm to 21mm isn't so different and it will still leave the graffiti detail quite soft I suspect.

Cheers,

[oldgreygary]

Unfortunately, with the G2 you cannot set the in between focal lengths with precision. The variable focal length is adjusted by a lever on the shutter button which moves it out and in. But, until you look at the metadata I don't know what the FL is.

I would agree to a certain extent about zooming in on the picture but I would also suspect that it is also related to how the camera performs in low light conditions.

When looking at the DOF calculator as an example it suggests that at f/2.8, FL 7mm at 50 metres then the nearest acceptable sharpness is 2.51m and farthest is infinity.

I took three images of a tape measure indoors at 7mm/21mm and a guess FL which was 9.2 the following are the images:

7mm



9.2mm


21mm


All three show reasonable sharpness at this shorter distance. Beyond the red 1' mark the sharpness seems to tail off fairly quickly. Perhaps, that is normal?

Cheers for now

Gary

[John 2]

You're introducing another variable here. The closer you focus, the smaller your depth of field. In terms of your original concern of the difference in quality in the train shot taken at some distance, this will not tell you anything. The camera is doing what it is supposed to. Incidentally, the quality looks OK for a small sensor camera of its generation.

[oldgreygary]

Yes John, I think your are right I am ending up confusing myself on this. I think that I just need to go out and take pictures! Otherwise, If already not, I am in danger of getting bogged down in too much technicality.

Considering that the camera is 10+ years old and is one of the earlier generation of digital cameras the quality is good. I guess that sometimes my expectation or vision of what sort of image I want can be sometimes let down by the limitations of the camera.

Cheers for now

Gary

[John 2]

It's a classic Gary. Canon have always led with their G series cameras and some manufacturers are only just catching up. You're right, there are some things that it won't tackle well but you just have choose your subject. There's a post of mine around at the moment (After the Storm)that was taken with a G7. I don't think an SLR would have made much difference had I used one. The main thing is to use it and enjoy it - and post the results here for some feedback.