Question on exposure compensation


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sfhuang

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#1
Just went for a studio shoot the other day and was dismayed that I could not use my Nikon 50mm f/1.4 lens. :bheart:

Reason was because the studio lights were pretty strong and as I was using an S1 Pro at ISO 320 (it can't go lower than 320) I needed to set my aperture to a minimum of f/22. This value was based on rough calculation from the metered value of f/11 at ISO 100 from the light meter.

Now here's the problem. The 50mm f/1.4 can only close down to f/16 and no further. I needed f/22. As I was rushing for time, I switched lens and didn't bother with reducing strength of the studio lights.

But now thinking about it, would it have produced the correct exposure if I used the lens at f/16 and compensated with -EV 1 stop? Logically this would work, but I'd like to hear from the experts. What are the side-effects of compensating for over-exposure like so? Loss of details? Less accurate colors? hmm.

Thanks.
sfhuang
 

chunsan

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#2
Hi,

I am no professional, but what I hear, I give you my humble opinion.

In my simple solution to you, use neutral density filter to reduce the powerful flash.

Secondly, you can use lower flash power, say at f8.

I am not versed in digital, but in film, over exposed by half to one stop is acceptably good, it get rid of grain

As for transparency, under by half stop is good.

Hope this will help you.
 

sfhuang

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#3
Originally posted by chunsan
Hi,

I am no professional, but what I hear, I give you my humble opinion.

In my simple solution to you, use neutral density filter to reduce the powerful flash.

Secondly, you can use lower flash power, say at f8.

I am not versed in digital, but in film, over exposed by half to one stop is acceptably good, it get rid of grain

As for transparency, under by half stop is good.

Hope this will help you.
will using a ND filter cause any degradation (e.g. loss of detail) of the final image as compared to if I lowered the flash?
 

roygoh

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#4
Using ND filter should have minimal impact on the image quality.

EV compensation means that the camera will adjust the shutter or aperture value (depending on aperture or shutter priority modes used respectively) from the metered value at EV 0 compensation.

So EV compensation is limited by the max/min aperture and shutter of the camera.

For example, in aperture priority mode, if the theoretical shutter speed required is 1/4000 sec (a very bright subject), and the camera's maximum shutter speed is 1/2000, using EV-1 compensation will not help as the camera simply cannot make any more compensation bu increasing the shutter speed.
 

roygoh

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#6
In the case of a studio shoot with flash as the main source of illumination, the actual exposure on the media (CCD in your case) is controlled by:

- ISO
- Aperture
- Flash output power

Shutter speed plays very little part because the main illumination is the flash, which fires in a very short duration (1/1000 ~ 1/10000 sec), and the ambient light is comparatively low.

In your case, you have reached the limit in ISO (320), and you have reached the limit in aperture (F/16), but the exposure is still too strong. So the only option is to reduce the flash power.

The camera does no have much control over the exposure. Unless the EV compensation can somehow reduce the ISO rating of trhe CCD from 320 to 100, which is quite unlikely, there is nothing much the camera metering can do over controlling the exposure.

The best thig to do is to request to reduce the power of the flash, if it was a group studio shot. The other photographers should be able to compensate for that 1 to 2 stop reduction in the flash power easily. If possible, I would prefer to shoot at somewhere around f/8 instead of the minimum aperture setting of f/16.

- Roy
 

roygoh

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#8
2 reasons, may or may not be valid to everyone:

1. I am under the impression that most lenses perform best when the aperture is somewhere between the maximum and the minimum setting.

2. I also prefer to have the background less in focus than the subject, to improve the subject-background separation. In a studio setting, f/8 should give adequate DOF to keep the subject in focus from eyes to ears.

:)
 

roygoh

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#10
Originally posted by Wai
since your lens cannot stop down to f22, how about increase the shutter speed instead?
Already explained. The bulk of the exposure is provided by the flash, which has a very brief exposure time only.

The shutter speed has to be at least slower or equal to the flash sync speed, and it does not have much impact on the exposure, as the ambient light is usually comparatively low.

If the flash output is too high, and the ISO and aperture are both at the limit setting already, there is nothing else that can be done except:

1. add a ND filter
2. Reduce the output power of the flash

- Roy
 

sfhuang

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#12
Thanks everyone (esp Roy for the excellent explanations!!). Really learnt a lot here. :thumbsup:

So the only effective way to get around the problem is to use a ND. If that's the case, is there a recommended exposure limit that you compensate with the ND? By this I mean, can I go as low as ND -3 ev ... this would allow me to reach f/8 to get the beautiful DOF that Roy described. :p

cheers,
sfhuang
 

roygoh

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#13
Originally posted by sfhuang
Thanks everyone (esp Roy for the excellent explanations!!). Really learnt a lot here. :thumbsup:
You're welcomed!

Originally posted by sfhuang
So the only effective way to get around the problem is to use a ND. If that's the case, is there a recommended exposure limit that you compensate with the ND? By this I mean, can I go as low as ND -3 ev ... this would allow me to reach f/8 to get the beautiful DOF that Roy described. :p

cheers,
sfhuang
I would say the most effective way is to reduce the flash output power if it is possible. If it is a studio flash it should have adjustable output power. Unless all the other photographers in the same session are using ISO 50 and f/22 that they need that kind of high power from the flash....

Using ND means the EV compensation has to be considered in the reading of the light meter.

ND comes in different strengths. You should be able to find one that gives you the required number of stops of attenuation. -3 stops = 1/8 trasnmission?

Also, if the ND you got is not really neutral, ie, it gives a colour cast, then you would want to do a custom WB setting with the ND filter attached.
 

#14
Originally posted by roygoh
2. I also prefer to have the background less in focus than the subject, to improve the subject-background separation. In a studio setting, f/8 should give adequate DOF to keep the subject in focus from eyes to ears.

:)

Not for medium format though. I once tried f/8 on a 6x7, 65mm lens (probably about 35mm in 35mm equivalent). Insufficient DoF. ;p

Regards
CK
 

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