Depth of field confusion

I've just bought a Canon Eos 20D digital SLR, as an upgrade from my Minolta Dimage 7i. My reason for posting here is because I'm concerned and confused that the depth of field in the 20D images is a lot shallower than from the 7i.

I understand the basics about smaller aperture = greater depth of field and vice versa, and that the further the subject the greater depth of field etc. However, with the 20D it seems I have to stop the lens right down to its minimum before acceptable front-to-back sharpness occurs, which of course means I have to use a longer exposure and/or higher ISO rating.

I'm due to take pictures at a fairly well-lit indoor exhibition next week, and I chose the 20D because it got good reviews on its low-light capability and I don't want to use flash. However, if I have to open up the lens enough to allow motion-freezing 1/250 sec exposures, I'm worried that the depth of field will be too shallow to be acceptable. I need images that are sharp from front to back.

The lenses I bought with the 20D are Sigma 18-50mm and 55-200mm (I've used the 18-50mm most in testing my new 20D). The Minolta's integral zoom lens is 28-200mm, but its depth of field is much greater than the Sigma lenses.

Can anyone shed any light on this confusing subject?

Sorry to ramble, thanks in advance :-)

James
London

Depth of field is an optical property of a focal length, a focus point and an aperture. Think of it as an equation - change one value and the result (DOF) changes.

Now a DSLR like that is a different format than 35mm - the angle of view of whatever lens you're using is 1.6x less than on a 35mm camera. In other words, a 50mm lens now looks like about an 80mm.

Something doesn't make sense though - you should get more depth of field if you use the same angle of view (which is a shorter focal length) while keeping the focus point and aperture the same. Are you shooting at the same ISO speed? If ISO400 film and ISO100 on the digital, then you've got two stops less aperture to deal with for the same shutter speed.

Your 7i like my A1 and A2 uses a smaller sensor than the 20D (I bought one too!).
So the focal length of the lenses is shorter for the same apparent image size.

While my A1 and A2 are marked 28-200 (35mm equivalent) they are actually only 7.2-50mm focal length. With that short focal length the depth of field is greater.

from http://www.wrotniak.net/photo/dof/
The depth of field of a digital camera with a lens of the 1:N focal length equivalence ratio at a given F-setting is the same as that of a 35 mm camera with a lens closed down to the aperture number of F multiplied by N.

So with the Minolta 50.8:200 is about 4, so f4 on the Minolta gives equivalent DoF distance to f16 on 35mm !

Thanks, guys, for your help. There's a glimmering of further understanding going on at my end ;-)

Clutching at straws, I've read the following today in a camera shop's brochure about Sigma lenses: "Lenses with a DC designation are designed only for the smaller size APS-C and 4.3RDS system digital SLRs". Both my lenses are marked DC - could this be the problem?

Thanks again,

James

Don't you just love guys who answer the question without doing a little research first? That would be me!

So now it makes sense. I assumed that your previous camera was 35mm film but didn't bother (obviously!) to see that it's a compact digital. So my theory was right, but Paul helped out better - since you were using a smaller focal length for the same angle of view, you had more depth of field.

I've usually found too much DOF to be a problem with small digitals, moreso than too little with DSLR's. Different shooting style, I suppose.

Once again, it's not the lenses that are a problem. A 50mm is a 50mm is a 50mm - it's the angle of view that the focal length sees that's the issue here. A 50mm lens would be much longer than anything you have on your compact digital camera (forget about 35mm equivalents because they'll just cause problems with DOF - they're only valid when talking about the all-important angle of view).

OK, I kind of understand what's going on now...

I think part of the problem is that I was so used to the long DOF from my Minolta, the shallow one from the Canon came as a bit of a surprise. But the more I use it, the more I remember how my original 35mm SLR (Praktica BX20 - long time and 4 digital upgrades ago!) performed. I'm just going to have to be ultra-careful with my focussing!

Anyway, thanks very much for your assistance in helping me understand what's going on - I appreciate it :-)

James

Quote:

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Originally Posted by argon

I've just bought a Canon Eos 20D digital SLR, as an upgrade from my Minolta Dimage 7i. My reason for posting here is because I'm concerned and confused that the depth of field in the 20D images is a lot shallower than from the 7i.

I understand the basics about smaller aperture = greater depth of field and vice versa, and that the further the subject the greater depth of field etc. However, with the 20D it seems I have to stop the lens right down to its minimum before acceptable front-to-back sharpness occurs, which of course means I have to use a longer exposure and/or higher ISO rating.

I'm due to take pictures at a fairly well-lit indoor exhibition next week, and I chose the 20D because it got good reviews on its low-light capability and I don't want to use flash. However, if I have to open up the lens enough to allow motion-freezing 1/250 sec exposures, I'm worried that the depth of field will be too shallow to be acceptable. I need images that are sharp from front to back.

The lenses I bought with the 20D are Sigma 18-50mm and 55-200mm (I've used the 18-50mm most in testing my new 20D). The Minolta's integral zoom lens is 28-200mm, but its depth of field is much greater than the Sigma lenses.

Can anyone shed any light on this confusing subject?

Sorry to ramble, thanks in advance :-)

James
London

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DOF depends upon the REAL focal length of the lens, not some mathematical "35mm equivalent." The focal length of the Dimage lens is 7.2 mm to about 51 mm. The DOF runs from extreme wide angle (oodles of DOF) to moderate DOF. The lenses for your D20 have focal lengths not even close to 7.2 mm.

here is a comprehensive guide to depth of field in the digital and 35mm domain

http://www.photo.net/learn/optics/dofdigital/