'extra' depth of field?...

[mats_hellman]

A subject I'm surprised I haven't read any opinions about is the

following:

 

In current digital slr's, most cameras have a 'crop factor' compared

to 35mm film. While cropping the field of vieuw that the lens

produces, the lense obviously doesn't 'crop' the depth of field that

a lens produces.

 

In other words, take a camera with a 1.5 'crop factor' (e.g. nikon

D70). To get a field of vieuw that you have with a film camera of

say a 50 mm lens, one needs to use a 33 mm lens (if that existed, so

lets take a 35mm lens for this example). A nikon D70 and a 35mm lens

give the same field of vieuw as a film camera with a 50 mm lens. BUT

THEY DON'T OFFER THE SAME DEPTH OF FIELD!

 

Now this in practice means that given you keep using the same field

of vieuws (as one can do by using dx lenses) as you did with film,

you start getting more depth of field in your photographs in

general, on average.

 

As I've just bought a D70 last summer, I've noticed how used I got

to the relationship between the depth of field and of vieuw in my

lenses. Now that this relationship changes when using my digital

body, I got thinking... what do I really want? I think I really like

a bit of extra "free" bit of depth of field. (just for your info I

have a 24mm, 50mm and a 80-200mm)

 

Anybody else out there having their ways of seeing being interupted

and stirred up a bit?

 

What are your thoughts on this subject?

[david_h._hartman]

One of the factors involved in DOF is the focal length. Since smaller formats uses shorter lens for the same angle of view as larger formats, smaller formats tend towards greater DOF.

[mats_hellman]

Thats my point...

 

So what does that do for your photography?

[biju_s]

... so to get same DOF you open the aperture if possible (new aperture = old aperture/(1.5)^2). That of course increases the shutter speed. looks like it is not possible to get same photo(perspective, DOF and shutter speed) with cropped dslr.

[david_h._hartman]

<em>"So what does that do for your photography?" --Mats

Hellman<br>

</em><br>

Well, the extra DOF can be good or it can be bad. It all depends

on the subject. If Nikon comes out with a 70/1.4 lens with creamy

background rendering in the f/1.4 to f/2.5 range and a nice price

that would cheer many. Also one does not have to give up one

format to use another. <br>

<br>

I own APS to 4x5" cameras and the use varies widely,

especially APS v. 4x5". Im toying with very pocketable

digital P&S as a replacement for Polaroid and much of my APS

use. My APS cameras are a couple of Canon Elph Jr.(s) and an Elph

Sport.<br>

<br>

This looks like a DX v. Full Frame debated so Ill get while

the getting is good.<br>

<br>

Dave.

[raczoliver]

Same thing happens when one moves from large format to medium, or from medium to 35mm, but even more pronounced. The difference between 35mm and a DSLR probably won't change your shooting style a lot, but have a look at large format camera lenses. Most of the ones I have seen start with f/5.6, probably because having larger aperture than that is just not practical. Even at F/5.6, a person would have the eyes in forus, but not the ears. The smallest apertures of these lenses are very small, in case you want to photograph with a great depth of fiel, and guess what goes with this......Without the use of a tripod 100% of the times, these cameras become useless. So I guess moving from large format to medium format does a lot to your shooting style, but I wouldn't consider the difference between 35mm and a DSLR with 1.5x cropping factor that serious.

[frank uhlig]

Biji,

 

be careful not to get too confused:

 

You said " ... so to get same DOF you open the aperture if possible (new aperture = old aperture/(1.5)^2). That of course increases the shutter speed."

 

First part is correct; the second sentence is nonsense: if you open the aperture, you need to lower the exposure time; say from f/8 to f/5.6 to get about the same DOF as on the film camera. But then the speed goes from 1/250 to 1/500 for the same amount of light onto the sensor as went onto the film before.

 

Oh, these technicalities. And I think noone worries much about a shorter exposure time, greater shutter speed, right(?), except for streaky waterfalls, ocean edges, ... But usually the extra shutter speed is welcome. So opening up in 1.5 crop factor DSLR shots acts like using about one ISO stop faster film on DSLRs (for the same DOF as on a film body).

[steve_muntz]

With "increase the shutter speed" he might have meant a faster shutter speed - but yes, using a wider lens will in practice give you more DOF for the same aperture.

 

I also agree that a little more DOF wide open is generally a good thing. Having only used this lens once, I'd guess that the 12-24 wide open at f4 would give you a lot more DOF than my 20-35 wide open at f2.8; most noticible close focused. Obviously you are losing a stop to use the 12-24, but that's the only Nikon (and fastest) choice out there at the moment - and you're gaining DOF too because of the focal length.

 

You could always use an ND filter to slow the shutter speed if needed (or adjust ISO).

[ilkka_nissila]

In general this is a good thing. I've used both a lot in the past 6 months and I definitely prefer more DOF in all but very unusual situations.

[john_stockdale2]

The smaller the format the smaller the focal length for a give angle of view and the more DoF.

 

Only when the resolution of the smaller format is limiting is this a problem.

 

For this reason, I moved from 4"x5" to MF.

[paul_benson2]

I think a lot of what's been said above confuses an otherwise simple issue. The smaller sensor on a DSLR doesn't change the DOF or the focal length at all, it just crops the image. So, if I put a 50mm lens on both a film SLR and a DSLR, then focus them both at the same point I'll get the exactly same DOF. Only when I print the images at the same size (ie. blow up the digital print) will I appear to have used a longer lens on the DSLR, and subsequently gained an apparent increase in DOF compared with the ?virtual? (but not ?real?) increase in focal length.

 

Put another way; using a 50mm lens on a DSLR isn?t the same as fitting a 75mm lens to a film camera. It?s actually only the same as taking a 35mm negative and scanning in the middle 67% of the image.