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09-06-2012, 04:10 AM   #1
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Why no EV in M mode? Also, DOF with long focal lengths

EV question:

Why is there no exposure compensation option in M mode? The EV button appears to do nothing in M mode. So far I've been relying exclusively on the exposure triad of ISO, shutter speed, and aperture to get proper exposure, but I've noticed that green button metering on my K200D usually underexposes a bit. I usually decrease shutter speed to compensate, because I want lower ISO and the aperture is usually set that way for DOF/composition reasons.

DOF and longer focal lengths:

Does focal length have an effect on DOF? In viewing some EXIF data for certain wildlife photographs (Baby Bighorn2.jpg photo - Ashley Hockenberry photos at pbase.com for example), I've seen very large numerical aperture values (e.g. f/8) with what looks like shallow DOF. Granted, this could be because the backgrounds are so far away, as I've noticed that subject distance and background distance have a great effect on subject DOF.

Thanks in advance.

09-06-2012, 04:16 AM - 1 Like   #2
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EV QUESTION
Because it doesn't make sense. The exposure compensation works my overexposing/underexposing the automatic controlled setting. Example :
In Av mode, you dial in +1 exposure compensation and the shutter speed will be one stop slower.
In Tv mode, you dial in +1 exposure compensation and the aperture will be one stop "more open"/brighter [i don't know the best way to say it]

In Manual mode you control everything, so what would the exposure compensation do? Nothing.

DOF and longer focal lengths
It does. If you get a 200mm 2.8 and frame a person, you'll get a nice defocus.
Here, for example :

That is a picture of my aunt using my Sigma 70-200 2.8 at longest focal length and 2.8 aperture.
If i had used a 24mm f2.8 and same framing, the background wouldn't be nearly as defocused.
09-06-2012, 04:21 AM   #3
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1) If I'm not mistaken there is EV compensation in the last models at least (K-5/K-7). This allows you to get a compensated setting each time you press the green button.
2) I'm no physicist but from what I read, and from pictures I saw, yes, a longer focal length at equal aperture (e.g. a face at f/8) will result in a much shallower DOF. Try and photograph something with your 35mm and your 135mm both at and see for yourself! Never stop experimenting
09-06-2012, 04:28 AM   #4
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There is a good discussion of EC and M mode here: https://www.pentaxforums.com/forums/pentax-k-5/191057-k5-exposure-compensation-manual-mode.html. In short, EC in M mode does have some indirect, non-obvious effects.

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--Anders.

09-06-2012, 04:31 AM   #5
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Hello!

The larger F number means the smaller aperture. So F8 refers provides larger DOF. The DOF depends on: aperture, focal length of lens and distance between the subject and the camera. The larger focal length, the shallower DOF. I suggest this DOF calculator Online Depth of Field Calculator to understand the issue.
In case of aperture F8 and focal length 135 mm the DOF will be 1.75 meters wide if you focus to the subject 10 meters from the camera. If the situation is the same (distance – 10 m, aperture F8) but the focal length is 50 mm, the DOF will be 21.4 meters wide. So the difference is very substantial. Try the DOF calculator! And you will understand the issue better.


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Alberts
09-06-2012, 04:33 AM   #6
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QuoteOriginally posted by victordeamorin Quote
Try and photograph something with your 35mm and your 135mm both at and see for yourself! Never stop experimenting
For even more experimentation, try focusing on something with your 35mm and 135mm at the same aperture, but with the shot composed exactly the same. Move far enough back with the 135mm so the shot looks exactly like the 35mm shot. The DOF will be the same.
09-06-2012, 04:33 AM   #7
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QuoteOriginally posted by GabrielFFontes Quote
EV QUESTION
Because it doesn't make sense. The exposure compensation works my overexposing/underexposing the automatic controlled setting. Example :
In Av mode, you dial in +1 exposure compensation and the shutter speed will be one stop slower.
In Tv mode, you dial in +1 exposure compensation and the aperture will be one stop "more open"/brighter [i don't know the best way to say it]

In Manual mode you control everything, so what would the exposure compensation do? Nothing.
Thanks for the explanation. The EV thing makes perfect sense now. Before learning this, I really had no idea what was going on in the camera when I set EV on one of the semi-auto modes like Av. I don't really use Av much anymore though, since I'd been favoring my M 50/1.7, and my new M 85/2 is going to be stuck on my body for a while
09-06-2012, 04:39 AM   #8
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QuoteOriginally posted by elliott Quote
For even more experimentation, try focusing on something with your 35mm and 135mm at the same aperture, but with the shot composed exactly the same. Move far enough back with the 135mm so the shot looks exactly like the 35mm shot. The DOF will be the same.
So it seems to me that a longer focal length has the effect of bringing you closer to the subject, if we're talking DOF (and also in the viewfinder as well). I know there would be perspective differences.

09-06-2012, 04:48 AM   #9
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QuoteOriginally posted by ChooseAName Quote
EV question:


DOF and longer focal lengths:

Does focal length have an effect on DOF? In viewing some EXIF data for certain wildlife photographs (Baby Bighorn2.jpg photo - Ashley Hockenberry photos at pbase.com for example), I've seen very large numerical aperture values (e.g. f/8) with what looks like shallow DOF. Granted, this could be because the backgrounds are so far away, as I've noticed that subject distance and background distance have a great effect on subject DOF.

Thanks in advance.
Actually DOF & blurring background are 2 different things. You get some surprising results. With same framing of a subject (same size of subject in frame) shorter focal lengths actually gives less DOF but less blurring of background compared to longer lengths at same aperture.

Look at this http://www.bobatkins.com/photography/technical/bokeh.html and skip down to the "Blur Quantity" section
09-06-2012, 04:56 AM   #10
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From what i know, thats how it works :
The bigger the DOF, the less blur you'll get.
If you have a 0,1m DOF thats pretty shallow, because you have a 10cm in focus area, if you have a 50m DOF, thats bigger, and therefore gives less blur, because you have a 50m area that is in focus.
09-06-2012, 05:43 AM   #11
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since every one has thrown in their $0.02 I'll add mine and soon you wioll have a whole $1.00 of advice and comments.

EV compensation in manual, and by this I mean green button metering only, has been an on again off again issue with pentax. My *istD has it, the K10D does not, the K7D and K5D do. Although there are many who argue manual is manual and you should be in full control of the settings, as some have pointed out, and I support this position, certain cameras and/or lenses when combined have consistent metering issues. The most obvious is the K10D and K20D, which have very poorly behaved metering (clear understatement here) The camera absolutely MUST know the true open aperture of the lens to meter correctly. without this data, metering is only correct at F5.6 (kit lens open aperture) faster lenses tend to under expose by up to 1-1.5 stops by F2, and slower lenses can over expose by up to 2 stops at F8-11, and above F11 over exposure drops so that it is only 0-1 stop by F32. due to this non linearity in metering, which is corrected by the knowledge of the open aperture through the contacts on A series lenses, if you add a tele converter that feeds lens data through, without modification, you get serious errors. My K10D needs -.7 EV comp with my Sigma 70-200F2.8 and a 1.4x TC, and needs -1.3 in EV comp with a 2x TC. the error is constant across all apertures, therefore when shooting manual, if I meter off something on the K10D, I need to consider every time, to correct the exposure, I would love to be able to dial it in. Other situations, such as wanting to shoot either high key or low key, would similarly want to have a preset EV comp, but the K10D does not allow it. We are not talking here about manual is manual so you make all the adjustments yourself, we are talking here about manual is meter RELIABLY and then shoot trusting the meter. EV comp in this case is a way to meter reliably.


DOF is a different beast. When changing focal lengths etc, there are two factors at play, We have the competing effects of differing perspective, with focal length and also depth of focus. both have an impact on the out of focus rendering behind a subject. From a purely mathematical basis, if you take a DOF calculator, a subject and 2 lenses, both at F4 for example, if you consider modifying shooting distance so that the principle subject is the same size in both shots, the DOF calculator will show virtually identical depth of field for the two situations. I did this calculation for someone who was doing a studio shoot, and wanted to change from a 50mm lens to a 135mm lens, and wanted to know the change in background spacing considering having the same depth of field. BUT, because perspective changes, the out of focus background using a longer lens is proportionally larger, making it appear to be more out of focus, than when using a shorter lens, which reduces the size of the background relitive to the subject. It may be just as out of focus, but because perspective has made it smaller, the effect is not as pleasing.
09-06-2012, 05:45 AM   #12
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EV should also affect flash power output in M-mode
09-06-2012, 06:00 AM   #13
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QuoteOriginally posted by 123jippo Quote
EV should also affect flash power output in M-mode
I don't agree with this statement. Source?
09-06-2012, 07:00 AM   #14
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I shoot with a K-x and find that my M series lens used in Manual mode tend towards a consistent 1/2 stop or so underexposed reading when doing momentary stop down metering with the green button. Having EC to apply a consistent +1/2 stop adjustment to the metered reading would be very useful. Currently I need to press the green button and then remember to immediately give the exposure time wheel a click to the left (or was that to the right??) A bit of a PITA. Don't think the K-x allows EC to work in Manual mode (or at least have never noticed a setting in the menus to enable it).
09-06-2012, 08:45 AM   #15
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EC does affect flash exposure on the K-30 at least.
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