Depth-of-field conversion with D-SLR?

[Byron Gale]

I understand that the size of the sensor in my D-SLR causes a lens to have 1.5x the normal "zoom"... i.e. a 50mm lens acts like 75mm.

Is there some conversion necessary to use the DOF scale on a lens, when mounted to a D-SLR? Or does the phenomenon not apply to that?

Byron

[Chuck S.]

Byron, I think the depth of field calculator at this website:

DOFMaster

would indicate that the answer is "Yes".

For instance, the calculator shows that a 55mm lens set at f/16 on a 35 mm film camera focused to 10 feet would have a total depth of field of 9.39 ft. The same camera mounted on a Canon 30D would have a depth of field of 6.61 feet.

[NMarti]

Thanks Chuck, that's something I absolutely cannot get my mind around. I have tried to understand it but it just doesn't sink into this old brain.

[Chuck S.]

Nancy, I don't have it totally surrounded either....

Upon further thought, it may be that the lens marks are still an accurate depiction of the near and far focus limits, but that they represent different distances when the lens is mounted on a DSLR vs. a film SLR. Guess I'll have to try to do the math......


......NAH!!!!

[troush]

I just try to remember if I want more depth of field I've got to have a higher F# - like F22 or F32. If I don't care (using Bryan Petersen's method) - then use F8 or so. Then, if I want the background out of focus, use the lowest number for the lens - like F4. I don't do a lot of landscapes yet, so I'm usually more worried about what's behind my subject being in/out of focus, though..

-Trish

[Byron Gale]

Thanks for that link, Chuck!

Here's a shot of my lens markings, and the calculator's readings for my equipment.



It doesn't appear to be too far off, looking at the f22 marks, but when compared to the 35mm values for the same lens...



...and it is clear that things are different.

The DOF on the DSLR begins farther from the camera, and is shallower.

Byron

[NMarti]

Byron
That's a great demonstration.

Chuck
Me - math? I'll go for point and luck first!
I do sort of follow Trish's rule of thumb but always have to stop and say smallest number or largest number. I then go with large number - large behind, small number small behind (sort of like clothing sizes)

[Byron Gale]

I agree that the general rules apply, as Trish outlines. But if I am manually setting the focus to have my DOF's leading edge coincide with my subject... I guess I'm going to have to come up with a way of calculating things.

Or - I could just run through a few iterations on the web site, and make myself a cheat-sheet!!

(I would like to understand what's happening, though)

Byron

[bayhli]

This is all pretty new to me as well, but the DOF Master has settings that you adjust for the type of "film" (digital, film, APS etc) and the circle of confusion (digital, APS, 35mm etc) for your individual camera. My understanding is that there are differences and for accurate DOF calculations you need to be using the appropriate settings.

There is a lot of information at the DOFMaster site that would probably answer your questions Byron, most of which went over my head.

... and people do indeed carry cheat sheets for their lenses, calculated from gizmos like the DOFMaster.

[Grant]

Oh my DOF is one the most confusing aspects of photography. While it would be nice to say that my camera has 35mm lens conversion of 1.6 therefore ...bla bla bla but it is much more complex than that. The formula is rather scary

f = D*M^2/2C(M+1)

M is in fact the inverse of you lens conversion so for 1.6 M is 1/1.6 or 0.625 but c changes with cameras for a 35 mm camera it is 0.025, for a dSLR it will be about 0.015, and for a itty bitty camera it will be maybe 0.005.

So now you are totally confused and so am I as it is not all that easy to get your head around.

In the old days of fixed lenses they engraved the scale onto the lens. The don't do that now because most lenses are zooms and those engravings wold be worthless.

There are two things you can do.

1) Download a good DOF calculator, there are a bazillion free ones out there for both Mac and PC. Use them to get to know how your camera works.

2) learn these rules.

* Larger F-numbers give greater depth of field.
* Shorter focal lengths give greater depth of field.
* Greater subject distance gives greater depth of field.
* Depth of field is greater behind the subject than in front.

3) (ok I lied there are three things) If you camera has a DOF preview button us it.

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