Distance Scale on a Crop Body

[Chip]

I'm trying to better understand hyperfocal distance.

I have some old Nikon glass with distance scales on them. My dSLR has a 1.5 crop factor.

My question is: do I multiply the distance on a lens' scale to get the correct depth that will be in acceptable focus?

For example (and I'm rounding here for simplicity), on a 28mm lens at f/11, if the distance scale says that everything from 4' to infinity is in focus, does that translate to everything from 6' to infinity being in focus on my camera?

Thanks!

[inkista]

Yup. Should work like that, but I tend to just use a DoF calculator instead. This assumes, however, that you also remembered to multiply the hyperfocal distance the DoF scale is telling you by 1.5 as well. The hyperfocal distance of a 28mm lens at f/11 on a full frame is NOT going to get you focus to infinity on a crop body.

[Chip]

inkista - Thanks!

I'm getting there, and your explanation sure helps.

But, now I'm confused by your last sentence, where you said "The hyperfocal distance of a 28mm lens at f/11 on a full frame is NOT going to get you focus to infinity on a crop body."

Does this mean that on my crop body SLR , I can't rely on the infinity part of the lens' distance scale to be accurate, and that I should rely instead on the DOF calculator? Or is it something entirely different?

Thanks again.

[vandergus]

This is probably gonna get hairy but I'll try to explain it with an example. Having a lens in hand will probably help.

Some numbers from dofmaster
FX: 28mm at f/11 -> hyperfocal distance = 7.67 ft
DX: 28mm at f/11 -> hyperfocal distance = 11.5 ft

First of all the calculations show that the hyperfocal distance for the same lens and same aperture on different formats changes. The hyperfocal distance on FX is 7.67 ft while the hyperfocal distance on DX is 11.5 ft.

So let's say you're trying to set the hyperfocal distance on your crop body using the distance scale on the lens. You have you're aperture set to f/11 so you line up the infinity mark with the f/11 mark on the lens barrel, which means you've set the focus distance to about 7.67 ft. But you're using a cropped sensor so the depth of field doesn't actually extend to infinity, but is closer to something like 4.6 ft - 23 ft. In order to achieve an infinite field of focus you have to move the focus distance out to around 11.5 ft. After you do that, look at where the infinity mark now lines up. It should be pretty close to the f/8 mark on the lens barrel. So the rule of thumb that I would use to account for the cropped sensor would be to use the next larger stop (smaller f-number) on the lens barrel to set the hyperfocal distance. If the aperture is set to f/8, set the infinity mark at f5.6. If the aperture is set to f/16, use f/11, etc.

As an additional note, it good to keep in mind that the lens marking should only be considered as rough guides. The calculations used to place them on the barrel assume a standard viewing condition for the final image (print size, and viewing distance). If your final presentation differs significantly from that, then the DOF scale may be inaccurate.

Hope this helps you figure out a little more.

[inkista]

What vandergus said.

Originally Posted by Chip

Does this mean that on my crop body SLR , I can't rely on the infinity part of the lens' distance scale to be accurate, and that I should rely instead on the DOF calculator?

That's pretty much what I do. The distance scale on the lens should still be accurate, but the DoF scale is only accurate for full frame.

[Chip]

vandergus and inkista - Many thanks!

And vandergus, that rule of thumb makes sense and should come in handy. It also happily means that I won't have to memorize another set of numbers. My brain is already on overload from all the passwords I try to keep in there.

I think I "get it." But, if you don't mind, I'll mull this over a while. And if I still don't understand or if I come up with a new question or 2, I just might bug you again for more. (If that's OK of course. )

Thanks again.