[Zazu]

Hi. My name is Zazu and I'm from South Africa. Been a Pentax user since film. Got a small collection of Pentax cameras and lenses and enjoy all of them.
I've got a question that I can not find an answer for anywhere.

On APS-C sensors the length of a lens (primes) changes according to the APS-C factor. In the case of Pentax it is a factor of 1.5. That mean that a 15mm lens become a 22.5mm. But the f - number also change............ a f4 will become a f6 (f5.6 adjusted). Now what happens with the Hyper focal adjustment for depth of field. Do you adjust your lens Hyper scale to 5.6 ??. Or has the scale on the lens been adjusted/calibrated to suit the 1.5 APS-C sensor. I have tried to use a FF lens on a APS-C sensor and adjusted the Hyper focal scale for the 'extended' depth of field......and it doesn't work. Did Pentax take this in consideration. If not, then the scale as indicated on the prime lenses is wrong.
Maybe I've got something wrong here ???
Love to see some comments

Regards

[Na Horuk]

Hey, welcome!

That mean that a 15mm lens become a 22.5mm. But the f - number also change............ a f4 will become a f6 (f5.6 adjusted).

I don't like to look at the "equivalence" as such. I know some people love it, but many of us think its just a needless complication.

I have tried to use a FF lens on a APS-C sensor and adjusted the Hyper focal scale for the 'extended' depth of field......and it doesn't work.

I have a personal theory on hyperfocal. The original definition of hyperfocal is the nearest focus that gives acceptable sharpness (aka DoF) all the way to infinity. But the problem word is acceptable. I think what was once acceptable on film is no longer acceptable on modern day digital cameras to a lot of photographers. This is because modern day digital sensors have much more resolution, and photos can be easily viewed at "1:1 zoom", where each pixel on the photo corresponds to one pixel on the monitor. This means that photos are captured with more sensitive technology and scrutinized at closer range. Our personal standards also go up, because we are often exposed to "ultra sharp, super detailed" photos, especially online, but also with HD TV and other media.

Another problem is that modern DA lenses have a very short focus throw (designed for fast AF, not for Manual focus) and often don't have distance scales at all, much less DoF scales. With many brands, even the distance scales on there are miscalibrated (so the label 3m, for example, does not actually focus 3m from the camera sensor, as it should). This makes hyperfocal, or zone focusing at all, very difficult if not impossible.

So here is my advice: You take that lens that you want to use in hyperfocal, set it to the aperture that you want to use, and then you do tests with it. Put the camera on tripod, use 2 sec timer, and you take a couple photos, only adjusting focus a little bit between shots. Then you look at the photos to find which focus setting is the one that is truly "hyperfocal" on your gear, according to your standards. Now you can either remember where to focus, or you can even mark it on the focus ring.
Also, hyperfocal is only possible with wide angle lenses, because they will give you a wider apparent DoF. Which lenses would you like to use hyperfocal focusing on?

[BrianR]

Welcome to the forums!

I have a personal theory on hyperfocal. The original definition of hyperfocal is the nearest focus that gives acceptable sharpness (aka DoF) all the way to infinity. But the problem word is acceptable. I think what was once acceptable on film is no longer acceptable on modern day digital cameras to a lot of photographers.

Good point, but you don't have to commit to anyone's standards when working out DoF or hyperfocal distance.

Here's a calculator that the OP can use to see the difference between hyperfocal distance for differetn sensor formats:

A Flexible Depth of Field Calculator

and you can hit 'show advanced' to work under the stanards of different viewing conditions (huge prints, view from uncomfortably close, etc) if you want to up your criterea for acceptably sharp. There are even more flexible calculators that allow the choice of arbitrary circles of confusion Lens Magnification and Depth of Field Calculator, or this downloadable program (for windows) VWDOF.

(Not to dismiss your suggestion of a practical test - that's always a great idea!)

[Na Horuk]

(Not to dismiss your suggestion of a practical test - that's always a great idea!)

Yeah, I recommend a test because it also accounts for miscalibrated distance scales, factory tolerances, and even the photographer's/audience personal standards, and so on. But that tool is a good place to start, as well

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Last edited by Na Horuk; 03-02-2016 at 02:53 PM.

[stevebrot]

But the problem word is acceptable. I think what was once acceptable on film is no longer acceptable on modern day digital cameras to a lot of photographers.

"Acceptable" has been quantitated to a constant value based on average visual acuity and a standard print size and viewing distance (IIRC, 8x10 print at about 18 inches)*. It is that value that is used as one of the terms when calculating depth of field. That being said, on digital people tend to defeat and disregard very usable depth of field by assessing focus at full resolution. Unfortunately, DOF rapidly approaches zero as magnification increases.


Steve

* I sort of pulled those numbers off the top of my head. The gist is a moderate size enlargement at a comfortable viewing distance. The calculation is for acceptable circle of confusion (CoC).

[Kerrowdown]

Welcome to the forum, I'll just let the more clever folk than me around here, answer your technical question.

[dms]

Hyperfocal distance (H) is a model that simplifies an otherwise complex situation. It is valid even if you don't care for the results--by that I mean you can decide to bias the results, but don't discard the model.

If you want to use it for 1.5X cropped sensor, just use the hyperfocal distance for full frame 35mm, at 1 stop more open than you are actually using. e.g. a 28mm FL at f/8 H is about 10 feet--so on Pentax DSLR (not K-1) you would focus on/set distance as 10 feet if you are shooting at f/11.

If you usually crop a lot, or print large (much bigger than about 8"x12"), you will probably want to try 2 stops different--i.e., shoot at f/16, w/ the above example. Or maybe you print smaller, or you just find you can get away with a larger opening--and maybe f/8 is fine.

[mee]

Hyperfocal distance (H) is a model that simplifies an otherwise complex situation. It is valid even if you don't care for the results--by that I mean you can decide to bias the results, but don't discard the model.

If you want to use it for 1.5X cropped sensor, just use the hyperfocal distance for full frame 35mm, at 1 stop more open than you are actually using. e.g. a 28mm FL at f/8 H is about 10 feet--so on Pentax DSLR (not K-1) you would focus on/set distance as 10 feet if you are shooting at f/11.

If you usually crop a lot, or print large (much bigger than about 8"x12"), you will probably want to try 2 stops different--i.e., shoot at f/16, w/ the above example. Or maybe you print smaller, or you just find you can get away with a larger opening--and maybe f/8 is fine.

The only issue I have with this is once you pass f/8 on digital crop, it seems most lenses begin to get softer. By f/16 the lens is usually soft enough to to be considered warm Brie (cheese).

[Unregistered User 8]

Hi Zazu. Welcome to the forum.

[carlopezzi104]

No " f " number will not change ...

---------- Post added 03-03-16 at 08:04 AM ----------

No " f " number will not change ...

[dms]

Yes f number changes--for following reason.
If you use a smaller sensor, you must increase the magnification to get the same size print. And that (same size print viewed from same distance) is the basis of hyperfocal distance. In this case by 150% on width and height, and that means you need to stop the lens down by an equivalent amount (1 stop), to retain the same depth of field.

[stevebrot]

Hyperfocal distance (H) is a model that simplifies an otherwise complex situation. It is valid even if you don't care for the results--by that I mean you can decide to bias the results, but don't discard the model.

Well put. I do a bit of street photography at times and usually with non-AF cameras. The general solution is to set the camera up for hyper-focal and concentrate on framing and timing.


Steve

[stevebrot]

Yes f number changes--for following reason.
If you use a smaller sensor, you must increase the magnification to get the same size print. And that (same size print viewed from same distance) is the basis of hyperfocal distance. In this case by 150% on width and height, and that means you need to stop the lens down by an equivalent amount (1 stop), to retain the same depth of field.

Hmmmmmm...Hmmmmmm...

Sort of true, but not quite...sort of...

The subject gets a little mind bending when you start fiddling with the details. When the equations are fully reduced, DOF hinges on final magnification to the viewing medium, viewing distance, and absolute (not relative) aperture. Discussions regarding relative aperture (f number) must be qualified by focal length which modifies magnification* and so on. Using a decent calculator help a little when attempting comparisons. I use the DoF Calc app for Android and have found it to be valid for general use at moderate distances.


Steve

* Things get even stranger with close focus where the effective focal length is often significantly greater than at infinity and the f number correspondingly higher.

[dms]

RE: Sort of true, but not quite...sort of...

Well yes, and yes the effect is very significant in the macro range, but for normal shooting distances the changes in H are not significant IMO.

In fact I actually use values for H that are not precisely correct, because I can remember them easier. And I can more easily mentally change values as I change FLs and f stops. e.g., the difference between using H=10 feet or 12 feet, is usually not significant.

[Zazu]

Thanks to all the welcome messages. I feel already at home here. Seem that my question sparked a bit of a debate and lots of answers to think about. I will do a tests based on some of the above suggestions and comments.
Regards