LR5 - effect of applying lens profiles

[davidedric]

Another Lightroom (5) question. Any feedback welcome.

LR has a profile correction for my Tamron 70-300 zoom. When I apply it, I expected to see the perspective change, maybe any vignetting corrected, and maybe CR reduced.

However, the profile also shifts the whole histogram towards the right, and the image brightens. Is this normal behaviour?

(The exposure slider in the Basic panel is unaffected)

Any thoughts welcome.

Dave

[Glenn NK]

Since vignetting is essentially a loss of light at the corners of the image (in lens tests, it's measured in stops), the correction boosts the exposure in these areas, so changing the histogram (shifting to the right = increasing exposure) would seem to be an expected outcome.

Glenn

[davidedric]

Thanks, Glen, that makes sense, but though I think there is some vignette correction going on, it isn't all that I can see.

To test this out, I have tried cropping down to the middle portion of the image, and applying lens correction does still visibly lighten the remaining image. I have even had blown highlights appearing in the middle of the image that weren't blown before!

It's nothing that I can't live with - it just seems odd.

I only have two lenses, so I am going to have a look at some of the images with my shorter zoom. Likely to be a better test, because I usually shoot the long zoom pretty much wide open to get decent shutter speed and ISO, whereas the short zoom is much more likely to be stopped down.

(Edit) Yes, the short zoom seems to behave the same way!

Dave

[Glenn NK]

Dave:

I don't know what lenses you have, but they may have been tested by Photozone.de:

http://www.photozone.de/canon_eos_ff

One of my lenses (Canon 17-55) has some pronounced vignetting at f/2.8 - almost 1 1/2 stops.

http://www.photozone.de/canon-eos/42...is_50d?start=1

As for cropping in Lightroom, I'm not sure that LR only "reads" the crop - IOW, I suspect it still reads the entire original image even though it's been cropped. But I defer to anyone that knows more about this than I do, and if someone knows the answer, please respond.

Glenn

EDIT:

I just looked at an image shot with my 17-55 at f/5.0, and although I didn't see any brightening when I applied the lens corrections, the histogram did shift noticeably.

Can't explain why I've never noticed that until now (after you've pointed it out).

[davidedric]

As for cropping in Lightroom, I'm not sure that LR only "reads" the crop - IOW, I suspect it still reads the entire original image even though it's been cropped. But I defer to anyone that knows more about this than I do, and if someone knows the answer, please respond.

So far as I can tell, by doing some drastic crops just to see the effect, I think that the LR histogram "reads" the crop - but like you I will defer to anyone who actually knows.

[Manfred M]

Don't forget Lightroom is a completely parametric editor; no pixel in the base image is ever altered, unlike Photoshop, where many of the functions are destructive. I don't use Lightroom, but rather Camera RAW, which is the base engine that runs Lightroom, so the results should be similar / identical. Crops, lens corrections, etc are based on lens profile data and this would be applied to any based uncropped image, and this will come through on the crops. I find that lens correction primarily removes distortions and vignetting, which means your image will lighten up as the darker corners are brightend up.

[RustBeltRaw]

I wouldn't be terribly surprised if lens correction targets are based on a known illuminant, in which case they may "correct" the exposure across the entire frame. For instance, if your glass doesn't let 100% of the light through (none do), then the lens correction profile is probably aware of this and may brighten the entire frame. If it already brightens the corners, why not the center as well?

It'd be interesting to take the same shot at the same exposure value with both a pro lens and an entry-level equivalent. The former should let more light through, so when the corrections are applied to both lenses, I would expect to see the entry-level lens's file brightened more than the pro's, even though they started at the same exposure. The difference is probably slight. Pity still lenses aren't rated with the same t-stop system as cinema lenses. Then I'd know if this theory has merit just by looking at the aperture ratings.

[dje]

Yes maybe the lens correction compensates for transmission loss of the lens as Lex suggests. If you look at some test results in DXOMark, you see that this loss is typically around half a stop.

Mind you, this seems like strange thing to do given that when taking the shot, the camera metering will have effectively allowed for this loss already.

Dave

PS : Just did a quick test in ACR. Enabling lens correction (Sigma 17-70) shifted the histogram slightly to the right and gave a small increase in brightness but nothing like half a stop. So much for that theory ! Also cropping does change the histogram.

[davidedric]

Thanks to Victoria Bampton (Lightroom Queen) I have the answer - it is down to vignetting

Enabling lens profile also enables the manual vignetting and distortion slider. The default for both is 100%. If you reset the vignetting slider to 0 the histogram shift goes away.

Also, a very tight crop, say 10%, of the centre of the image does not show the histogram shift.

Dave