Newbie Question Colour saturation for a scientific experiment

I have to produce a seriese of about 150 colour prints for an experiment.
In the experiment people will be shown the pictures and given another stimuli, I cant go into details here (untill its published), but it should give some information on how people decide what food looks appatizing and how that depends on colour.

So i have some pictures that i have to change the colour of. To do that is very simple, i just shift the hue so many degrees round the colour wheel (in photoshop).

There is a problem though. When i take a picture of a bright red apple and change the hue by +36 degrees there is a very big change in the colour of the picture. When I take a picture of a rotting carrot and change the hue by + 36 degrees it only looks a bit more green, because rotting carrots are actually pretty much greys (Less saturation).

so the visual change imparted by a change in colour is dependent on the saturation and the lightness of the subject matter.

I think I can change the Lightness of my subjects quite easily in photoshop, by applying levels (setting the average value of all the greys in a picture to a value) but how do i do the equivilant for saturation? I need to make sure that all of my pictures have the same amount of colour in them irrispective of what that colour is.

Anyone got any ideas?

For that matter how to get rid of the tedium of adjusting cropping and otherwise making a load of ugly edditing on sooo many pics may help too!

tpe

I'm not shure if I know what you mean, but I'd say use the hue/saturation command. You can set color, saturation and lightness there. (You should have the colorize box checked)

Why not try using image/adjust/color balance ?
There's options in there for shadows, midtones and highlights.....

image/adust/variations would work also