DoF and 1.5x: Correct me if I'm wrong ...

Take a lens, any lens.

At specific settings it will have a specific depth of field.

Now let's say for argument's sake that the first image captures part of a scene say 10m wide, and it has a depth of field of 1m. This depth of field is roughly 10% (1/10) of the width of the captured scene.

Now I move from my P30n film camera to my K10D with its crop factor of ≈1.5. My K10D captures part of the same scene equivalent to let's say 7m - the actual number is irrelevant but it's important to note that it is LESS than the original width. The same lens at the same settings has been used, so the depth of field would still be 1m. This means the new image has a depth of field ≈14% (1/7) of the width of the captured scene, which is an increase over the original.

Now these numbers are hypothetical, but I believe that the DoF:width ratio increased.

By my calcs, an image taken with a 77mm on the K10D @f4.0 will have more than twice the DoF an image taken with a 115mm on my P30n @f4.0 has, even though they capture an almost equivalent field of view.

I'd like to hear some thoughts because I read something by someone (intentionally vague) that said the opposite.

It's not quite as simple as you've made out

You need to read up on "The Circle of Confusion" - which is, alas, a little vague and subjective. There's a very nice online DoF calculator (make sure you enter the actual focal length, not your 1.5x - because the focal length never actually changes... it's the angle of view that changes )

http://www.dofmaster.com/dofjs.html

The bottom line is: it is different to 35mm, and you can do nothing about it.

Of course, to actually compare like for like is difficult - to get the same image on the sensor means you have to be further away with digital, or use a wider lens on digital - both of which change the calculations.

I never give it a second thought - just click away and work harder with digital to get what I want
Matt

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http://www.mattmatic.co.uk
(For gallery, tips and links)

It always amazes me how often this issue pop's up at various forums.
As a general rule of thumbs you can say:
With the use of a fixed focal lenght and a fixed aperture DOF will increase as film( or sensor) size decreases.
One example: Ever wondered why small digital compact never have an aperture range above f8 ?And large format cameras go to even f128?
Generally speaking you can say when you compare different filmformats(or sensor) goiing from digital compact thru Dslr(cropped) to Dslr (FF) to Medium format to Large format etc ,etc....you have to stop down about two stops to gain the same DOF given the use of the same focal lenght.
Hope his helps
martin.

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Best regards,

Martin.


Curious about my photography?? Just Follow the Light.

That's close, but don't forget that a , say, 50mm lens is always a 50mm lens regardless of what size sensor you put behind it. And at a given distance will always have the same DOF.

What changes is the sensor size, so on a small format it will be a telephoto with the same DOF that it will be on a large format, but in the latter case it will be a standard or wide angle lens.

If you take the "standard lens" for a format, it will have a different focal length depending on format size and small formats (110 = 24mm) will have more DOF than large formats (6x6cm = 80mm) for the same field of view. THis is what leads us to believe that more DOF is there with small sensors, and for practical purposes there is.

Other factors also come into this, but it's too early to go into all that now!

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Best regards, John

I ran the calcs with the two different circles of confusion for 35mm and APS-C digital sensor, and the ratio of the depths of field came out as the square of the crop factor. So basically the digital camera with the 77mm focal length would have about 1.52² times the depth of field a 35mm film camera would have with a 115mm focal length at the same aperture.

It's not precise because I'm not doing a thesis, but the net result is increased depth of field on my K10D over my P30n.

Michael,
It's not as bad as that

As I indicated, you have to take into account the changed angle of view. So, let's assume you are trying to get the same image in 35mm and K10D:
* Subject at 2m
* K10D with 50mm lens
* 35mm with 75mm lens (to provide the same AoV)

K10D DoF = 0.25m
35mm DoF = 0.16m

So, it's actually not the square, but approximately the AoV multiplier.
(If you used the same lens, from the same distance, then it would be the square... but the image would be wider on 35mm and therefore not a good comparison).

Confusion, confusion, confusion....
Matt

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http://www.mattmatic.co.uk
(For gallery, tips and links)

Let me think aloud a bit, if I may...

It is not the camera that makes the depth of field different. It's the fact that you're using a shorter focal length lens with APS-C.

It seems like you're saying that to get the same angle of view with digital, you need a shorter lens, so the DOF is greater. Nothing particularly controversial there, as far as I understand it.

However, the image size is smaller with your 77mm lens on APS-C, than the 155mm gives you on 35mm.

So the APS-C image will need more enlargement to give the same size print, than the corresponding 35mm image.

Presumably this is 1.5x as much enlargement.

Is this anywhere near right?

Yes.

So the APS-C lens will need higher resolution for the same degree of enlargement, which is borne out by lens design.

This is why test figures for superb medium format lenses seem very low when compared to 35mm format lenses.

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Best regards, John

Ok, then if Matt's right, and (at the same distance and aperture) the depth of field with the shorter lens is 1.5x greater (on the image), but you have to enlarge it 1.5x as much, doesn't this cancel out the additional DOF you get with the shorter lens?


This has always puzzled me. I thought the whole point of medium format was that it was for much bigger enlargements than 35mm.

If you only needed the same size print as on 35mm, then I could see why medium format lenses wouldn't need to be so sharp. But since the prints are supposed to be so much bigger, why aren't they equally sharp?

There are other factors involved - the Circle of Confusion will be smaller for the smaller format lens, which should equalise that out and still give us the extra DOF. Don't forget that we are looking at Real World situations here and this is what we see on the finished print. Try it and see.


If the enlargement sizes are the same then the larger format needs less enlargement than the smaller format to give a similar degree of sharpness. That is, about 4 lpmm on the print. This is true for every size of print. What also changes though is that the grain on the film is less apparent and the gradation is smoother and the resolution is not so far short that it negates the advantages.

Looked at another way, the 35mm print is sharp and a bigger 6x6cm print will be as sharp. There is enough resolution to deliver what is required for the eye to perceive on the print. I think 4 lpmm is the standard required for viewing at 1 metre.

The other factor is viewing distance. You don't generally view 20" wide prints at 1 metre, but much further away. Unless of course you are a certain type of Camera Club Judge, and then anything is possible...

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Best regards, John

For anyone who is still confused the link below is well written and what is more has a nifty little depth of field calculator.

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

Steve

Ok.

I forgot to mention in my post that my reference to depth of field should actually be APPARENT depth of field, or the ratio of depth of field to angle of view.

Thanks all.

Just to add my 2p worth as someone who uses depth of field tables and scales on an almost daily basis.

The figures vary enormously between manufacturers supplied tables, third party supplied ones which, for me are printed in the American Cinematographer Handbook, and the ones occasionally on the lens itself and this is all down to the circle of confusion size used by whoever compiled the table. Use the most pessimistic one you can find and remember that depth of field is in the eye of the beholder.

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Ken
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -

As a rule of thumb if the lens is set to, say, f11 I use the DOF figures for f8.

If we are very pedantic, there is no DOF. The only sharp plane is the one focused on....

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Best regards, John

And to be even more pedantic in that sense, everything is either behind the focal plane or in front of it. It's like the tenses: everything is either in the past or in the future, because whatever is in the "present" has already happened...