PhotoShop Autolevelling
Discussion
Could someone explain to me the basics behind the autolevelling in PhotoShop?
Why, will it sometimes not to anything or very little to a picture and on other occasions change it completely.
I took a few pictures t'other day and two of these were the ones which got me confused.
First one....
Original:
Auto Levelled (and slightly cropped):
And the picture that really confused me....
Original:
Auto Levelled:
There's no other tweaking of that 2nd shot bar Ctrl + Shift + L.
Why is the difference from the original so huge?
Cheers,
Graham
Why, will it sometimes not to anything or very little to a picture and on other occasions change it completely.
I took a few pictures t'other day and two of these were the ones which got me confused.
First one....
Original:
Auto Levelled (and slightly cropped):
And the picture that really confused me....
Original:
Auto Levelled:
There's no other tweaking of that 2nd shot bar Ctrl + Shift + L.
Why is the difference from the original so huge?
Cheers,
Graham
Auto Levels is basically "Levels" with certain parameters set in the "Options" screen (assuming you're using PS and not PS Elements).
The default settings for this is generally regarded as not being particlarly good (the % setting it uses is too high), so I always adjust levels manually by doing the following:
With your image open, open the levels dialogue.
What you should see in a well exposed picture is a histogram which looks like an upsidedown U which spreads across most of the window, but doesn't quite touch the sides. If everything is scrunched up at the left then the image is very dark (i.e. underexposed) and if it's over to the right, it's very light (overexposed). Obviously, a photo of a black cat in a coal cellar will have the histogram all the way over to the left, so this rule of thumb doesn't always apply.
What you do next is to select each colour in the dropdown list in turn (e.g., do Red, then Green then Blue instead of just RGB), and slide the left and right arrows in until you just start clipping the histogram.
You can see what pixels are being clipped by holding down the Alt key (on a PC) at the same time that you slide, and this shows the pixels that will be clipped. Basically what happens is that all pixels up to the point of the arrow will be set to either 0 or 255 depending on whether you're sliding the left or right slider. The remaining levels then get spread out across the histogram.
I generally move the slider until I can start to make out detail in the clipped display, then go back slightly, but it does depend on the image.
If you do this for each of the 3 colours, you should see the image become more contrasty, and by selecting/deselecting the "Preview" tick box, you can see what effect it has on the image.
If the histogram for one particular colour is much different to the others, then using the above method can change the overall colour of the image as you've found with the 2nd example.
In this case, you can try moving the slider for the 3rd colour to a similar position to the other two (based on the Input Levels values) , which should retain the original colour.
Give levels a try, and see what different settings do. I have always found that doing it manually gives better results than simply hitting Auto Levels, and if you don't like the result, just hold down "Alt" and hit "Reset", and try it again!
The default settings for this is generally regarded as not being particlarly good (the % setting it uses is too high), so I always adjust levels manually by doing the following:
With your image open, open the levels dialogue.
What you should see in a well exposed picture is a histogram which looks like an upsidedown U which spreads across most of the window, but doesn't quite touch the sides. If everything is scrunched up at the left then the image is very dark (i.e. underexposed) and if it's over to the right, it's very light (overexposed). Obviously, a photo of a black cat in a coal cellar will have the histogram all the way over to the left, so this rule of thumb doesn't always apply.
What you do next is to select each colour in the dropdown list in turn (e.g., do Red, then Green then Blue instead of just RGB), and slide the left and right arrows in until you just start clipping the histogram.
You can see what pixels are being clipped by holding down the Alt key (on a PC) at the same time that you slide, and this shows the pixels that will be clipped. Basically what happens is that all pixels up to the point of the arrow will be set to either 0 or 255 depending on whether you're sliding the left or right slider. The remaining levels then get spread out across the histogram.
I generally move the slider until I can start to make out detail in the clipped display, then go back slightly, but it does depend on the image.
If you do this for each of the 3 colours, you should see the image become more contrasty, and by selecting/deselecting the "Preview" tick box, you can see what effect it has on the image.
If the histogram for one particular colour is much different to the others, then using the above method can change the overall colour of the image as you've found with the 2nd example.
In this case, you can try moving the slider for the 3rd colour to a similar position to the other two (based on the Input Levels values) , which should retain the original colour.
Give levels a try, and see what different settings do. I have always found that doing it manually gives better results than simply hitting Auto Levels, and if you don't like the result, just hold down "Alt" and hit "Reset", and try it again!
Try hitting Ctrl-L to show the Levels dialogue.
Using the drop-down list at the top you'll see that the histograms representing how much red and green is in the picture span the whole width, whereas the blue only reaches half-way across the X-axis.
This means you've got no blue in the highlights, but a fair bit in the shadows (water).
With blue selected, drag the right-hand triangle left to the end of the graph, and you'll see the blue image data spread across the whole brightness spectrum - effectively adding blue to the highlights as there was none there before.
This is what auto-levels is doing - it has to assume there's an equal proportion of red, green and blue in the shot, which combine to make white.
In my experience you get better results by bringing the shadow and highlight triangles in to the edges of the RGB histogram than you do by using auto-levels. This maximises contrast, and you can then move the middle slider to alter overall brightness under RGB, and colour balance by selecting one of the individual colours.
In your shot, you've got everything from yellow to black, meaning you physically can't get any more contrast or colour information in this picture (well done!) so personally, I'd leave it as it is!
Dan
The first rule of photography: always have a camera with you!
Using the drop-down list at the top you'll see that the histograms representing how much red and green is in the picture span the whole width, whereas the blue only reaches half-way across the X-axis.
This means you've got no blue in the highlights, but a fair bit in the shadows (water).
With blue selected, drag the right-hand triangle left to the end of the graph, and you'll see the blue image data spread across the whole brightness spectrum - effectively adding blue to the highlights as there was none there before.
This is what auto-levels is doing - it has to assume there's an equal proportion of red, green and blue in the shot, which combine to make white.
In my experience you get better results by bringing the shadow and highlight triangles in to the edges of the RGB histogram than you do by using auto-levels. This maximises contrast, and you can then move the middle slider to alter overall brightness under RGB, and colour balance by selecting one of the individual colours.
In your shot, you've got everything from yellow to black, meaning you physically can't get any more contrast or colour information in this picture (well done!) so personally, I'd leave it as it is!
Dan
The first rule of photography: always have a camera with you!
ehasler said:
Stuff
I said:
What he said...
Everything I've read suggests that using autolevels is a big no-no, all the Photography mags pan it as being too crude and in fact can ruin a perfectly good image. For what its worth, I'd say the original image is better than the auto-levelled in both instances.
>> Edited by trackdemon on Tuesday 21st December 15:26
I found that when using cheap and nasty digital cameras, Auto levels made a massive change to all the photos, but since using the 300D it's never made anything more than a very slight change.
I love the auto-corrected pictures posted above! They look much better than the originals to me
I'll keep all that technical mumbo jumbo above for reference when next using PS though.
I love the auto-corrected pictures posted above! They look much better than the originals to me
I'll keep all that technical mumbo jumbo above for reference when next using PS though.
Wow, well that's given me something to play with, thanks guys, very good explanations and I understand it better now, I think in future I'll just tweak the levels to how I want it rather than being lazy and letting the computer do it
Steve, I know what you mean, the first image I thought was a little dull for some reason and the auto level just gave it a little more colour. The 2nd image I actually like the original and just wanted to see what AL would do, I can't make my mind up about which one I prefer, I feel the water in the auto levelled shot makes the reeds stand out better but I don't particularly like what the auto levelling did to the sun, and I like the golden tones in the original.
Thanks Phil, I know what you mean about auto levelling with a crappy camera, when using it with my old cam it actually used to make the pictures look kind of good, but since I got the D70 it's not really been having much of an effect.
Many thanks,
Graham
Steve, I know what you mean, the first image I thought was a little dull for some reason and the auto level just gave it a little more colour. The 2nd image I actually like the original and just wanted to see what AL would do, I can't make my mind up about which one I prefer, I feel the water in the auto levelled shot makes the reeds stand out better but I don't particularly like what the auto levelling did to the sun, and I like the golden tones in the original.
Thanks Phil, I know what you mean about auto levelling with a crappy camera, when using it with my old cam it actually used to make the pictures look kind of good, but since I got the D70 it's not really been having much of an effect.
Many thanks,
Graham
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