How to edit photos with different exposures so that they look the same? Please advise

[JayG99]

Hi all,

I am a dental student and I'm improving my dental photography.

Since I'm a newbie - my main problem is getting consistency in the exposure. I will try and improve my photography technique.

However, I have 2 photos here of the same case, taken on different dates (before and after), and they have different exposure - can anyone help to teach me what adjustments to make to make the exposure the same, and therefore the colour of the gums and teeth and lips to be consistent across both photos?

Any help greatly appreciated!

 

[Tom Mann]

Re: How to edit photos with different exposures so that they look the same? Please ad

A family friend is a dentist, and it happens that I went through this exact discussion with him a few years ago. Here is a quick summary:

1. Most importantly, don't immediately turn to PS to save you. Instead, first concentrate on exactly reproducing the multiple exposures (presumably spaced by months or years apart) in the camera. To do this, I suggest you dedicate a camera and lighting system (eg, ring light around the lens) and leave it set up in your office for you to use whenever needed. I would further suggest that you attach a small rod (ruler, etc) to the tripod mount on the bottom of the camera to exactly reproduce the distance between the lens, light and the subject. At the distances you will be shooting, a change of just an inch in distance will make a profound difference in illumination.

Turn the camera and flash to full manual mode and come up with settings for f-stop, ISO, shutter speed, white balance, and flash output that you will never change.

If your exam room has windows to the outside world, the fraction of light from them will vary with the time of day and season. Close the blinds to counteract this.

If you exam room has fluorescent ceiling lights, turn them off because their brightness and color temperature varies significantly at twice the power mains frequency, and your exposures won't occur at the same exact point in each cycle of the mains voltage.

Don't use your ordinary exam light for photography -- it's location, aiming, color temperature and brightness all vary and will be another uncontrolled variable. Instead, get a ring flash to mount on the end of your lens.

Get a MacBeth color checker card. Take a picture of it (exactly the same way as described above) immediately before you photograph each patient. You can use these images to iron out any small remaining differences in brightness, colors, etc. I'll describe how in a later post.

2. Only after you have done all of the above, should you start to think about post processing techniques.

If you are interested, I'll describe them in subsequent posts. However, if there is one point to get across, most importantly, shoot only in RAW, not JPGs. You will be able to achieve vastly better brightness and color matching using the adjustments in your favorite raw converter software compared to trying to color correct an image that's already rendered to a JPG or a TIFF or any other conventional image format. Generate the needed JPGs (say, for printing, for viewing by your patients, publications, etc.) afterwards.

More later. Gotta go.

Tom

 

[JayG99]

Re: How to edit photos with different exposures so that they look the same? Please ad

Hi Tom,

Firstly, thank you so much for taking time out of your day to post such a helpful reply, I really appreciate it!

I 100% agree with you - the prevention of inconsistent exposure is far better than the cure. I will work towards using your techniques described to focus on getting it right. At the moment I shoot in JPEG - I will try to now shoot in RAW mode - thank you.

I use a ring-flash at the moment (Canon MR-14EX) but I think the reason the exposures were inconsistent in the above photos was because, as you mentioned, an inch or so of difference in magnification which had a profound effect on lighting.

I will look in to a MacBeth color checker card also.

If you do get the time, I would still be interested in learning any processing techniques you may know which may be able to dig me out of a hole if this happens again.

Thank you and happy new year!

 

[MikeMc]

Re: How to edit photos with different exposures so that they look the same? Please ad

Tripod with the ring light, mark a spot on the floor for the same distance each shot, You can also use an 18% grey card for exposure consistency.

Also agree with shooting in RAW...the camera can have a mind of its own at times

Side note....Great repair of that chip!

 

[sprucemagoo1]

Re: How to edit photos with different exposures so that they look the same? Please ad

You could do with a jig in a room with no other light source. Very much like ex-raying the teeth.

 

[Tom Mann]

Re: How to edit photos with different exposures so that they look the same? Please ad

@Spruce - I like your idea. Although a bit more complicated than my simple-minded suggestion to mount a rod/ruler under the camera to exactly reproduce a given distance, it has the advantage that nothing actually even comes close to touching their face.

With the right camera (ie, one with "focus trapping"), another possibility is use a manual focus lens, tape the focusing ring to the desired distance, and turn on "focus trapping". This means that you hold down the shutter release button, slowly move back and forth, closer and further from the patient, and only when you are at the correct (fully reproducible) distance will the camera actually take a picture.

@JayG - Sorry for the delay in responding, but I'm still dealing with some major, time-consuming computer problems here.

With respect to your question: "...I would still be interested in learning any processing techniques you may know which may be able to dig me out of a hole if this happens again..."

Here's a quick technique that you could use to try to make two images taken under somewhat different conditions to be as consistent as possible: Assuming that the skin does not change brightness or color between your two shots, try to minimize the difference between the skin tones in the two shots. An easy way to do this is to load both images into one PS file. Put the older image (the reference image) on its own layer above the more recent image. Set the blending mode for the upper layer to either "subtract" or "difference".

Make copies of both images and blur these copies by the same amount - enough so that pores, blemishes, etc are no longer visible, but you can still see the major features of the face. My guess is that a radius around 100 or 200 pixels would be fine. Turn off the originals.

If the two blurred images are fairly close in brightness, contrast, color, etc., what you should see is a fairly dark image (because the differences between them is small).

Now, introduce adjustment layers between the two images and play with them to further darken the averaged skin areas in the difference image that you see. At minimum, I would suggest that you introduce a "levels" adjustment layer, and a "vibrance / saturation" adjustment layer. When you have minimized the differences between the blurred images, turn them off and turn the originals back on. Leave the adjustment layers on.You should now have a nicely tweaked version of the newer image.

Let me know if you have any questions about this technique. There is also an old thread on this difference technique. When I get a chance, I'll try to find it for you.

HTH,

Tom

 

[Tom Mann]

Re: How to edit photos with different exposures so that they look the same? Please ad

And here's a quick demo of the PP'ing technique i just described as applied to the two images posted by the OP.

T

 

[Tom Mann]

Re: How to edit photos with different exposures so that they look the same? Please ad

BTW, the sequence in the above animation is:

1. Black
2. Before treatment, before color correction.
3. Before treatment, after color correction.
4. After treatment.

Also, when I wrote up the description of the technique, I hadn't really looked carefully at the pictures you posted. When I looked at them more carefully, I realized that in addition to brightness, contrast and saturation differences, there was also a substantial color balance difference, so I had to include a "color balance" adjustment layer in addition to the adjustment layers I suggested in my previous post.

You will find that it takes a fair amount of experience performing all of these adjustments to get the darkest difference image, ie, the best match between the skin tones on the two images. Don't expect to be able to go through the adjustments once and come up with a good match -- you'll likely have to iterate through all the adjustments a few times to really get a good match.

HTH,

Tom