Do compact digital cameras have a smaller depth of field and if they do, why?

Do compact digital camera have a smaller depth of field range and if they do, why?

Can anyone explain if they do.
The aperture range on the compact is 2.8 to 8.
on my SLR it's 2.8 to 22
yet I am being told depth of field on a compact is great.

I am not understanding something fundamental here... can you help?

Gayesart,

Welcome to the Cambridge in Colour forums.
To help you with your question, it's all about sensor size. I suggest you have a look at this Cic tutorial. This is one of the many tutorials you can find on this site, and they are very useful for understanding photographic matters.
I refer to this tutorial because I think it explains the issue much better than I could. There also may be some other forum members they will reply (with good ability to explain this matter).
After reading, you may ask anything you like over here.

Hi, Gayesart, and welcome! Is Gayesart the name you like to use, or is there a first name we should use?

One thing to keep in mind, I think, while reading: the meaning of the statement about compact cameras is 1) the minimum depth of field of a compact sensor is greater than a bigger sensor across the range of use; and 2) the depth of field for a smaller sensor will be less in the same shooting situation.

Here are examples you can check with the DOF calculator in the DOF tutorial.

Take a compact camera with a 1/1.8 sensor, lens f/2.8 @ 6mm, f/5.6 @ 20mm. Compare a DSLR with a 1.6 crop sensor and a lens f/4 @ 18mm, f/5.6 @ 55mm. These don't correspond to any specific cameras, but they're pretty typical.

Say we focus on subjects 3m away with lens wide, 10m away with lens at tele. If you run all this information through the DOF calculators, you see that the compact has DOF = infinity @ 6mm focused at 3m, DOF = 130m @ 20mm focused at 10m. By contrast, the DSLR has DOF = 9.6m @ 18mm focused at 3m, DOF = 8.5m @ 55mm focused at 10m. You can see that the DSLR has far tighter DOF.

Another (better) way to compare is to look at an equivalent shooting situation. This is the basis of the calculator in the tutorial Jeroen referenced. If we want to fill the frame with a subject, we need a specific combination of focal length, distance, and sensor size. If I want to take a picture of my cat with a compact camera all the way tele, I'll have to be a different distance away to fill the frame as with a DSLR all the way tele. This tutorial has a calculator for focal length. You can use it to verify that for a 5m subject, I'll be 14m away shooting with a 20mm lens on a 1/1.8 sensor. And for the same 5m subject, I'll be 12.3m away shooting with a 55mm lens on a 1.6 sensor. Now if we plug these back into the DOF calculator, we see that DOF = 14m @ 55mm, f/5.6 focused at 12.3m for the DSLR, and DOF = infinity @ 20mm, f/5.6, focused at 14m for the compact.

Cheers,
Rick

Hi Gayesart,

To answer what I think you are asking is a compact does have a great depth of field for sharp front-to-back images but does not compare with the shallow depth-of-field offered by larger sensors and lenses on the DLSR. So the sales pitch is partially correct just not informative.
Everything said above I agree with and I too recommend you read the tutorials and remember to ask again if there is still something you don't understand.

Gaye is fine. I'm a digital artist and my business was called Gayesart for a while.
Thankyou for your responses. It will take me a while to work through your answers and tutorials.
I think in pictures, and the picture I have is the distance between the sensor and the lens in a compact digital is smaller. The bending power of the lens therefore has to be greater to bend the light rays onto the sensor (focus points).
Take a standard portrait shot,
The steeper angle and less space means the focus point for the face is close to the focus point for the background so you wont get blurred backgrounds.
The background will be acceptably sharp. With a longer distance between lens and sensor, it wont be. Circles of confusion will be bigger at the point of contact to the sensor (background to face) and so blurred.
My problem is the maths given to me does not create a picture... no picture... don't understand.
This site, amongst all the others I have been looking at impresses me and I trust your answers.
I appreciate the time spent answering my question.

Hi Gaye
I fessed up here in a recent post that I too struggle with maths. The picture you paint above is correct so I am not sure you need to understand the maths as long as you can understand and visualise the concept, which you appear to have done.