Question about changing f stop exposure for class

[shyy]

Hi guys,


I have a project for my photography class. We are using regular B&W film camera. For this project we have to show a subject in different exposure which shows 3 stops over and under exposed. My question is if my logic is correct.
Say I am using my Canon AE-1 Program and it meters and tells me the subject correct exposure is f2 at iso 400 with a shutter speed of 125. My prime lens f stop range goes from f2 to f4. To get the first 3 under exposed, I would use the following parameters.
-3 = would be a F0.7 but since my lens can only do f2 - I would change my shutter speed to three stop faster which is 1/1000?
-2 = would be a F1 but since my lens can only do f2 - I would change my shutter speed to two stop faster which is 1/500?
-1 = would be a F1.4 but since my lens can only do f2 - I would change my shutter speed to one stop faster which is 1/250?
n = 2 from Canon's meter so I would leave it at f2 with the shutter speed of 125
1 = 2.8, so I would just adjust it one stop to f2.8 with the shutter speed of 125
2 = 4, so I would just adjust it two stop to f4 with the shutter speed of 125
3 = 5.6, so I would just adjust it three stop to 5.6 with the shutter speed of 125


Does my logic seem correct?


Thanks!

 

[Sherm]

Sounds correct to me but hopefully someone with a lot more knowledge will come by and confirm.

 

[shyy]

Hey Sherm,

Someone corrected me, I have it in reverse

I believe this is the correct one

-3 = would be a F5.6 but since my lens can only do f4 - I would change my shutter speed to one stop faster which is 1/250?
-2 = would be a F4 I would leave my shutter speed to 125
-1 = would be a F2.8 I would leave my shutter speed to 125
n = 2 from Canon's meter so I would leave it at f2 with the shutter speed of 125
+1 = 1.4 but since my lens can only do f2 - I would change my shutter speed to one stop slower which is 1/60?
+2 = 1 but since my lens can only do f2 - I would change my shutter speed to two stop slower which is 1/30?
+3 = 0.7 but since my lens can only do f2 - I would change my shutter speed to three stop slower which is 1/15?

 

[snowbear117 48]

You corrected as I was typing.

 

[Patrice]

The premiss is that correct exposure is f/2 at 1/125 with constraints as follows: iso constant at 400 and lens limited to f/2, f/2.8 and f/4

equivalent exposures to the correct one are: f/2.8 at 1/60 and f/4 at 1/30

1 ev underexposed: f/2.8 at 1/125 or f/2 at 1/250
2 ev underexposed: f/4 at 1/125 or f/2.8 at 1/250 or f/2 at 1/500
3 ev underexposed: f/4 at 1/250 or f/2.8 at 1/500 or f/2 at 1/1000

1 ev overexposed: f/2 at 1/60 or f/2.8 at 1/60 of f/4 at 1/30
2 ev overexposed: f/2 at 1/30 or f/2.8 at 1/15
3 ev overexposed: f/2 at 1/15

As you can see there are quite a few possibilities even with only being able to manipulate the shutter and an aperture constrained from f/2 to f/4.

Closing the aperture from f/2 to f/2.8 reduces exposure by 1 stop as does decreasing shutter time from 1/125 to 1/250. The reverse happens when you either increase the aperture by a stop , ie from f/4 to f/2.8, or by increasing the shutter open time from 1/125 to 1/60 as an example.

edit: corrected the typo (fraction shown in red)

 

[BrianV]

>> "Closing the aperture from f/2 to f/2.8 reduces exposure by 1 stop as does decreasing shutter time from 1/125 to 1/500."

Changing the shutter speed from 1/125th of a second to 1/500th of a second causes a 2 stop change in exposure.

Another way to do the exercise: change the film speed dialed into the camera.

For ISO 400 film:

50 Three Stop Overexposure
100 Two Stop Over-exposure
200 One Stop OVER-Exposure
400 ACTUAL exposure
800 One Stop UNDER
1600 Two Stop UNder
3200 Three Stop UNDER

 

[pgriz]

>> "Closing the aperture from f/2 to f/2.8 reduces exposure by 1 stop as does decreasing shutter time from 1/125 to 1/500."

Changing the shutter speed from 1/125th of a second to 1/500th of a second causes a 2 stop change in exposure.

Another way to do the exercise: change the film speed dialted into the camera.

For ISO 400 film:

50 Three Stop Overexposure
100 Two Stop Over-exposure
200 One Stop OVER-Exposure
400 ACTUAL exposure
800 One Stop UNDER
1600 Two Stop UNder
3200 Three Stop UNDER

Could you explain how changing the film speed setting on a film camera would give you a change in exposure? I can see that working for a digital camera, but ....:scratch:

 

[ann]

Because it is the same either way. ISO values have the same relative value. The faster the ISO the less light needed, the lower the ISO the more light that is needed.

 

[o hey tyler]

>> "Closing the aperture from f/2 to f/2.8 reduces exposure by 1 stop as does decreasing shutter time from 1/125 to 1/500."

Changing the shutter speed from 1/125th of a second to 1/500th of a second causes a 2 stop change in exposure.

Another way to do the exercise: change the film speed dialted into the camera.

For ISO 400 film:

50 Three Stop Overexposure
100 Two Stop Over-exposure
200 One Stop OVER-Exposure
400 ACTUAL exposure
800 One Stop UNDER
1600 Two Stop UNder
3200 Three Stop UNDER

Could you explain how changing the film speed setting on a film camera would give you a change in exposure? I can see that working for a digital camera, but ....:scratch:

If you're shooting ASA 400 film, and you bias the camera to read ASA 200, it's going to expose one stop over than what it should for ASA 400. If the camera has a meter that uses what ASA it's set to, its essentially biasing the camera. I'm not sure in what application you'd want to overexpose though.

 

[Patrice]

>> "Closing the aperture from f/2 to f/2.8 reduces exposure by 1 stop as does decreasing shutter time from 1/125 to 1/500."

----- should have been 1/250


Thanks for pointing out the typo. Post has been corrected.

 

[curtyoungblood]

>> "Closing the aperture from f/2 to f/2.8 reduces exposure by 1 stop as does decreasing shutter time from 1/125 to 1/500."

Changing the shutter speed from 1/125th of a second to 1/500th of a second causes a 2 stop change in exposure.

Another way to do the exercise: change the film speed dialted into the camera.

For ISO 400 film:

50 Three Stop Overexposure
100 Two Stop Over-exposure
200 One Stop OVER-Exposure
400 ACTUAL exposure
800 One Stop UNDER
1600 Two Stop UNder
3200 Three Stop UNDER

This isn't really going to work on a film camera like it does on a digital camera. Changing the ISO on the camera doesn't do anything to the film, it is still ISO 400. If you set the ISO to 800 AND re-meter to a correct reading, then you will have an image that is under-exposed one stop. You would get the exact same effect if you just got a correct reading and then adjusted the shutter speed or aperture to under-expose a stop.

 

[Helen B]

>> "Closing the aperture from f/2 to f/2.8 reduces exposure by 1 stop as does decreasing shutter time from 1/125 to 1/500."

Changing the shutter speed from 1/125th of a second to 1/500th of a second causes a 2 stop change in exposure.

Another way to do the exercise: change the film speed dialted into the camera.

For ISO 400 film:

50 Three Stop Overexposure
100 Two Stop Over-exposure
200 One Stop OVER-Exposure
400 ACTUAL exposure
800 One Stop UNDER
1600 Two Stop UNder
3200 Three Stop UNDER

This isn't really going to work on a film camera like it does on a digital camera. Changing the ISO on the camera doesn't do anything to the film, it is still ISO 400. If you set the ISO to 800 AND re-meter to a correct reading, then you will have an image that is under-exposed one stop. You would get the exact same effect if you just got a correct reading and then adjusted the shutter speed or aperture to under-expose a stop.

That's the point of Brian's suggestion. This discussion is about deliberate under- and over-exposure.

 

[curtyoungblood]

I just want to make sure it is clear that changing the ISO setting on a film camera isn't going to change the exposure, like it would on a digital camera.

For example, if you take a meter reading, change the ISO, and take a photo, then the picture is still going to be properly exposed.

 

[ann]

You would over expose when wanting to make white snow white, would be one example as the meter is going to underexpose as it reads it as middle gray value.

 

[TCampbell]

The only reason a film camera has an ASA/ISO setting is to let you tell the built-in meter (for models that actually have a built-in meter) what film speed is currently loaded. The exposures suggested by the built-in meter are based on that setting.

Yes, technically you could lie to the camera and report that you've loaded a different speed than is actually in the camera and this would cause the meter to lie to you in return about what setting to use. But why do that? You already know you plan to take the meter's advice for the middle exposure and then deliberately run the camera up and down 3 stops in each direction. Many film photographers can share at least one story where they changed film types, forgot to tell the meter, and shot an entire roll at the wrong setting <sigh> guilty <sigh>. Numerous cameras have a holder to let you slip in the end of the cardboard box that the film came in -- which tells you what film is in the camera (lest you put the camera down for a few days before picking it up again and then can't remember which film is in it.)

For this class exercise, I'd probably pick a scene that already requires a LOT of dynamic range. For example... a landscape scene on a sunny day will often need 6 stops of dynamic range. This gives you a subject where you can really see the impact of over exposing and underexposing a shot.

If you over-expose and under-expose by shutter speed alone then the depth of field wont change... just the amount of light. If you use the f-stop to vary the exposure then you'll see a difference in depth of field as you vary the exposure (which is why on digital cameras, if you plan to merge the images for HDR you would normally want to preserve the aperture so that the depth of field is the same in all images.)

Good luck!