locking exposure

[Harmless`]

I need to lock exposure for a series of photos that are taken 1 1/2 hrs after the first. I'm trying to get a true representation of before and after shots of teeth whitening. The problem is that the after shot makes the skin look lighter. The procedure is very successful, and I don't want any artificial enhancements. This seems to be a problem for all metering modes and I'm wondering is this because I have to be quite tight with the shot and the new lightness of the teeth is the cause.
I'm using a 7d with 100mm macro and ring flash

 

[Phil V]

I need to lock exposure for a series of photos that are taken 1 1/2 hrs after the first. I'm trying to get a true representation of before and after shots of teeth whitening. The problem is that the after shot makes the skin look lighter. The procedure is very successful, and I don't want any artificial enhancements. This seems to be a problem for all metering modes and I'm wondering is this because I have to be quite tight with the shot and the new lightness of the teeth is the cause.
I'm using a 7d with 100mm macro and ring flash

I'm a semi auto kind of bloke; but exp lock is for a recompose - shoot scenario.

If you're shooting minutes apart, you'll be better to use Manual, set it for the first shot and forget it (presuming the light source is constant).

If you're lighting it with flash, set that manually too.

 

[D4rr3n]

Hi Chris

Manual everything would seem to be the answer.
ISO
Aperture
Shutter Speed
Flash Power

I'm no expert with flash (especially macro), but I'd start with setting my preferred iso, aperture and shutter and manually adjust the flash power for a good exposure.

If the flash can only get you close you'll need to adjust aperture to get the exposure spot on.

leave those settings on the camera (I'd make a quick note of them as well) and the other shots should be exposed as the first.

Hopefully one of the macro guys or girls will be able to advise as to how critical distance to subject will be re exposure.

 

[HoppyUK]

Use manual, note camera settings, and keep them the same. As Phil says, it's vital to keep the light exactly the same and any change in distance with any artificial light source will change exposure. It's very sensitive at close range.

If there are any slight changes, adjust final exposure in post processing using the skin tone as reference.

Edit: Crossed post with Darren

 

[DuncanDisorderly]

If you're shooting minutes apart, you'll be better to use Manual, set it for the first shot and forget it (presuming the light source is constant).

If you're lighting it with flash, set that manually too.

also...
On the Canon bodies, make sure you set ISO manually too.

I quite often forget. If I've been using auto ISO and put the camera in M and forget to set ISO; it leaves the camera no option to 'correct' my exposure than by changing the ISO - results are often stupid ISO and not what I wanted :bang:

Edit - lots of similar responses - you get the general idea

 

[Harmless`]

I am shooting on manual and all the settings are the same, no auto anything except the exposure and flash, the flash is on ETTL. I've never played with the flash , just attached and fired. Maybe that's the problem. Thanks.
Lighting is pretty stable its a windowless room with overhead lighting

 

[Ivan S]

Example of photo's might be needed to determine what is happening then with the exif data included.

 

[Phil V]

I am shooting on manual and all the settings are the same, no auto anything except the exposure and flash, the flash is on ETTL. I've never played with the flash , just attached and fired. Maybe that's the problem. Thanks.
Lighting is pretty stable its a windowless room with overhead lighting

Your exposure isn't manual if the flash is ettl.

But what does no auto anything except the exposure mean. The exposure is either manual or auto, if its auto that'll cause problems too. Like we all said, manual everything or you'll never get the shots.

 

[Harmless`]

Your exposure isn't manual if the flash is ettl.

But what does no auto anything except the exposure mean. The exposure is either manual or auto, if its auto that'll cause problems too. Like we all said, manual everything or you'll never get the shots.

Just exposed a massive whole in my understanding of my camera and probably exposure and what that means!
I take the shots in manual mode with a fixed aperture, shutter speed ,ISO and white balance. These are not changed during the shoot. The ring flash is on ETTL, which is, as you say, not fixed so I shall change that to a fixed value. Which metering mode do you think I should use, as I am now using centre weighted.
I don't think I understand the difference between automatic exposure and manual exposure and don't know have to switch from one to the other. I know how to change the exposure, but I don't in these cases as I really want the only difference to be the tooth colour, but it seems the flash may be the problem.

I haven't included any photos as patient confidentiality doesn't allow this until I change the consent form.

 

[Phil V]

Just exposed a massive whole in my understanding of my camera and probably exposure and what that means!
I take the shots in manual mode with a fixed aperture, shutter speed ,ISO and white balance. These are not changed during the shoot. The ring flash is on ETTL, which is, as you say, not fixed so I shall change that to a fixed value. Which metering mode do you think I should use, as I am now using centre weighted.
I don't think I understand the difference between automatic exposure and manual exposure and don't know have to switch from one to the other. I know how to change the exposure, but I don't in these cases as I really want the only difference to be the tooth colour, but it seems the flash may be the problem.

I haven't included any photos as patient confidentiality doesn't allow this until I change the consent form.

You were exposing Manually (apart from the flash).

Despite what you believe about the ambient, it will change over hours (*my windowlit office can change 2 stops in 2 mins), so you need the ringflash to be your primary light source. Set the Manual exposure to be at least 2 stops under, then adjust the flash power till the exposures are right. Metering mode is unimportant, your exposure will only be measured by the histogram when you're setting the flash power.

 

[Harmless`]

Ok, thanks a lot.
I'll have to practice as this isn't a photoshoot and I've got to be quick as the patient won't want to sit around waiting for me to get it right. When I get it right I'll be happy to put it in my portfolio. Thanks for the help

 

[KIPAX]

I take the shots in manual mode with a fixed aperture, shutter speed ,ISO and white balance. These are not changed during the shoot. The ring flash is on ETTL, which is, as you say, not fixed so I shall change that to a fixed value. Which metering mode do you think I should use, as I am now using centre weighted..

None... if your in Manual the metering mode does nothing. Once you set your camera up in manual and you have the correct exposure you require.. the most important thing to do then.. is to totally ignore whatever the exposure bar is showing.

 

[Harmless`]

None... if your in Manual the metering mode does nothing. Once you set your camera up in manual and you have the correct exposure you require.. the most important thing to do then.. is to totally ignore whatever the exposure bar is showing.

ok, thanks that why its the same which ever I use.

 

[FrattonFreak]

lighting - macro or standard the science is the same - light reduces inveresly square proportional to the distance light moved away

Sounds complicated but its easy

per unit light moved away from subject light reduces

1 unit away, light power = 1
2 units away, light power = 1/4 (2 squared)
3 units away, light power = 1/9 (3 squared)
4 units away, light power = 1/16....you know the rest

 

[FrattonFreak]

In terms of your dilemma why not use say a grey card in the image or colour chart and match images with the constants

Firstly shoot semi auto say aperture and note aperture (you set) and camera set shutter speed.

Set manual matching above - adjust exposure to ensure card colours match

 

[Harmless`]

In terms of your dilemma why not use say a grey card in the image or colour chart and match images with the constants

Firstly shoot semi auto say aperture and note aperture (you set) and camera set shutter speed.

Set manual matching above - adjust exposure to ensure card colours match

Its a good idea exempt for the circumstances. You have to imagine that your dentist is just taking a record photo of your teeth/mouth quickly. Its going to look pretty unusual with a colour chart, though some of my photographer patients may appreciate it!

 

[StewartR]

In terms of your dilemma why not use say a grey card in the image or colour chart and match images with the constants

Its a good idea exempt for the circumstances. You have to imagine that your dentist is just taking a record photo of your teeth/mouth quickly. Its going to look pretty unusual with a colour chart, though some of my photographer patients may appreciate it!

I think *all* patients would appreciate it if you briefly explained that it's necessary to ensure accurate reproduction of colours from one photo to the next. It would send a useful subliminal message.