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07-19-2009, 08:58 AM   #1
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Using DOF scale on old A series primes?

As some of you know I've just entered into using manual focus having just bought both a 50mm & 28mm A series lens. It would be nice to be able to use the DOF scales but I guess with the cropped sensor of the K20D this isn't going to work correctly.

How do people tend to use these manual lenses, particularly for landscapes, on the "A" setting or in full manual ?



07-19-2009, 10:26 AM   #2
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If the A setting is there, you might as well use it. It's easier for the camera and you. In a few specialized cases, using the aperture ring is better, but in general it just makes things more difficult with no benefit. Depending on your camera and lens, the exposures might be different too. The A setting is likely to be more accurate.

On APS-C, the DOF is larger than the scales imply. I pay no attention to this difference because I'm rarely perfect in estimating distance to subject. The extra DOF is just insurance.

A story about my dad: he had a Pentax P3 film camera that he was having trouble with, so I gave him a ZX-M (MZ-M in metric ). He had a Pentax-A 50mm f2.0 lens for it. He took a roll of film with it and said it was fine, but his trifocals made it hard to focus, so he was using the DOF scales to focus. That's fine with him because he is used to it. Then my sister gave him a newer Canon film camera, and after using that, he says it is amazing. "It doesn't have those scales, but somehow the photos are perfectly focused every time! Your sister says it worked like that for her too. " I visit and discover that the lens says USM on it. I had to explain the technology to him.
07-19-2009, 10:37 AM   #3
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usually you have to stop down one more stop compared to what f stop guide numbers you are using for focusing.
So, if you focus using f5.6 numbers on your lens, shoot at f8 and the focus will be (should be) fine.
As far as A setting goes, I see no reason not to use it.
PS: is the Hoya 28 with A setting???

07-19-2009, 10:37 AM   #4

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Plus 1 on always using the 'A' for correct metering and matrix metering option.
But you can still use the DOF markings on the lens, taking the crop factor into account. You can do that by applying the factor to the chosen F stop (if you are a calculus genius ).

07-19-2009, 10:40 AM   #5
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QuoteOriginally posted by Just1MoreDave Quote
On APS-C, the DOF is larger than the scales imply. I pay no attention to this difference because I'm rarely perfect in estimating distance to subject. The extra DOF is just insurance.
Oh dear, more DOF controversy.

For the a given focal length and final enlarged image size, the smaller (APS-C) sensor gives you less depth of field (you're enlarging the blur more because the sensor image is smaller).

I use the same rule of thumb as Axl: Use the DOF scale markings for one stop faster than the aperture setting (if you're taking an image at f/8, use the DOF scale markings for f/5.6).

Also, keep in mind that these rules apply to prints of around 8x12 inches. Larger prints have a smaller apparent DOF.

Last edited by troyz; 07-19-2009 at 10:55 AM. Reason: typo
07-19-2009, 11:46 AM   #6
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I'm pretty sure the DOF scale isn't changed, even on a crop sensor.
07-19-2009, 02:12 PM   #7
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The DOF scales are not that far off. You can probably use them with good success without too much problem. After all, consider the precision of the meter scale on the lens barrel itself!

There are several DOF calculators on the Web that allow you to dial in pretty much any combination of focal length, aperture, subject distance, sensor size, and final image size. Using those, you can make your own table to tape to the inside of your camera bag. One of the coolest things you can do is to make your own hyperfocal table and turn your SLR into fixed focus, fixed aperture point & shoot.

A few things to consider:
  • The scales on the camera are an estimate based on moderate final viewing size
  • Big final images have less DOF than pocket-sized prints
  • Your focus screen will show more DOF than you really have
  • The notion of DOF is purely subjective. In reality, only one point in the range is in focus regardless of aperture.


(Survived the DOF wars on this site from last year...)
07-19-2009, 03:07 PM   #8
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Thanks for the info guys but to be honest I'm pretty well up to speed with DOF/Hyperfocal distance (I carry around customised charts).

My question was more related to the "operation" of manual lenses and whether there was a different technique the sounds of it I guess not, which makes sense anyway, not having used them before I was probably just thinking about things too much



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