How to use the Light Meter accurately and effectively

[khoa_nguyen1]

Hello all,

 

I had been using grey card for the last 5 years. I finally got a Konica VF auto

Light Meter. I remembered someone told me in the past that if I use the light

meter as incident meter, I would need to adjust a exposes few stops to account

for the lost of light through the lense.

 

This theory does make sense to me; but I am not sure the accuracy of the

theory; or if it is true, howmany stop I should adjut for. I assume that the

adjusted f-stop would varies with each lense.

 

FYI, I own the following Lenses

Nikon 17mm-28mm/f2.8 -

Nikon 35mm-70mm/2.8 -

Nikon 50mm/f1.4 -

Nikon 105mm/f2.0 -

Nikon 135mm/f3.5 -

Nikon 80mm-200mm/2.8 -

LEICA D VARIO?ELMARIT 2.8-3.5/14-50 mm (for Digilux 3).

 

Please let me know if the adjustment f-stop theory is true.

If yes, please help me to figure out the adjusted f-stop's for each lense

above. If you could, please explain to and help me to understand how you

figure the adjusted f-stop.

 

Also, I am welcome any tip, trick or instruction how to use the light meter

accuratelly and effectively.

 

Any help is greatly appreciated.

 

Khoa

[kevinteo]

There is always going to be a difference; I don't think any lens at f1.4 is also T1.4. But I find

that the difference is usually not an issue with most negative films. Just remember that when

not metering through the lens, you will also need to consider filter factor of anything else

that you have on the lens.

[Hector Javkin]

There is a small difference between F-stops (aperture calculated as a ratio of lens diameter and focal length) and T-stops (the actual transmission of light) but it's nothing like a couple of stops. Even a zoom lens with 17 elements will hardly diminish the amount of light getting through. Most of the loss will be a result of internal reflections, but coated lenses minimize those.

 

The only lenses I know of that show a large difference in amount of light (close to one stop, sometimes even more) are mirror lenses that have a big obstruction in the front for the secondary mirror.

[lex_jenkins]

If the lenses are accurately calibrated you won't need to make any adjustments based on the assumption that there is a difference between what the light meter tells you and the actual transmitted light.

 

But...

 

As Hector points out, transmission and f/stops don't always agree. Also, it's not unusual for a lens to vary 1/3 EV, plus or minus, at each aperture, from actual transmission. Some lenses vary considerably more than that. For all but the most critical exposures, 1/3 isn't enough to worry about. But it's a good idea to test your lenses at all apertures under controlled conditions to check exposure accuracy and consistency.

 

You'll find some disagreement over what a light meter is telling you. Conventional wisdom claims 18% reflectance. Not necessarily so. This is based on the standard mostly applied to sensitometry evolved from the Zone System. According to ISO standards, light meters are calibrated closer to 12-12.5% reflectance.

 

Depending on whether you're metering using reflected or incident light, and the subject matter, it may or may not be appropriate to set your shutter speed and aperture differently from the reading given by the meter.

 

For example, lighter skin tones will be underexposed if the reflected metering reading is used as-is. It's usually more appropriate to open up a full stop, depending on skin tone. Conversely, when photographing dark skinned people it's often appropriate to stop down to avoid overexposing their skin. You can easily test this on white walls, black coffee mugs, and mid-tone objects that are middling green (the human eye can usually match the approximate reflectance of green and grey better than with other colors).

 

With incident metering, measuring the light source itself from the subject location, it's usually okay to use the exposure setting suggested by the meter. In years of b&w photography I seldom found that a spot meter gave me any different readings than an incident meter. Only when the subject was under significantly different light did a spot meter help - for example, if a distant mountain was in sunlight while the field I was standing in was overcast.

[fullmetalphotograper]

Here is a simple diagram on how to use an <a

href="http://www.ralphberrettphotography.com/FullMetalPhotographer.com/Techniques/Ent

ries/2008/1/2_Incident-Light_Metering.html" target="_blank">incident meter</a>. I hope

this helps<br><a href="http://www.ralphberrettphotography.com">Ralph

Berrett</a><br><a href="http://www.fullmetalphotographer.com">FMP</a><br>

[khoa_nguyen1]

Thank you all so very much for your responses and time.

 

Kevin, you mention that the difference is usually not an issue with negative films. Would the difference be noticeable or significant with the slide or digital, which I use? And how many stop should I adjust for a UV and Polarize filters? Is there a formula or method to calculate this?

 

Hector, do you have a method or formula to compute the differences between F-stops (aperture calculated as a ratio of lens diameter and focal length) and T-stops (the actual transmission of light)?

 

Lex, what do you mean by - if the lenses are accurately calibrated? -Does this mean that manufactures have accounted for the loss of light through lenses and for the actual transmitted light to reach the film, slide or cell sufficiently? How is this possible?

 

Lex, in regarding with testing my lenses, I am thinking of setting up a light source - measure the light source using the incident light meter - place a grey card directly under the light source and measure the reflected light from the grey card using spot meter of my cameras - Compare the results of the two readings - The differences between the two measures would be the adjusted Stop. Your thought please? and any suggestion would be greatly appreciated.

 

Lex, while we are on the calibrating subject, I have a question about calibrating of my camera. I am reading the Zone System. It frequently mentions that the Zone System only works with a calibrated camera. I have not calibrated my cameras since I bought them. How do I know if my cameras have been calibrated for Zone System? And how do I calibrate them? Do I need professional help to calibrate them?

 

Ralph, the FullmetalPhotography is great. Where should I place the incident meter if I use the digital camera to take B&W photo?

 

I love photography and trying to learn the subject; please be patient with me and forgive my ignorant of the subject.

 

Sincerely,

Khoa

[kevinteo]

<p>As Hector and Lex have pointed out, the difference between ƒ and T stops is

usually not more than ⅓ stop variance. Some films like Ektachrome Infrared have

relatively narrow exposure latitude, which may require that slight amount of compensation.

That said, I tend to bracket exposures just to safe.</p>

 

<p>Filter factor for UV filters is usually 1X (i.e. no adjustment is necessary). For a polarizer,

it may vary depending on the brand/type, but an adjustment of 1⅓ to 2 stops is

normal. See <a

href="http://en.wikipedia.org/wiki/Filter_factor">http://en.wikipedia.org/wiki/Filter_factor

</a> for more information.</p>

[SCL]

If you're being as detailed as the questions you ask, then it stands to reason that you need to know your actual shutter speeds rather than the indicated speeds. A shutter speed tester will give you a reasonable number you can use...the main issue is whether or not your shutter speeds are out of tolerance, which is generally 1/3 of a stop. Will that actually matter on slide film...it depends on how accurately you meter. You can initially bracket exposures and use the results to guide you in future compensation to your meter/aperture/shutter selections. If you shoot primarily outdoors normal subjects without wild swings of contrast, many people have been able to produce fully acceptable results by using the sunny 16 rule (no meter). If this is beyond you at present, when using slide film, meter off the highlights and you should do fine. You also mentioned digital, and the answer here is pretty simple...take a shot, look at the histogram (or chimp), and make the adjustments for your actual shot....you can do it right on the spot. There's no excuse for botched exposures in digital, unless you're involved in shooting situations which exceed the latitude of your sensor, then there are a variety of solutions involvin post processing.

[James G. Dainis]

Khoa,

 

I'm sure if you test the lenes as you suggested, using the incident light meter and a gray card, you will find very little, if any, difference. Do it and put your mind at ease.

[fullmetalphotograper]

Khoa, It does change the position of the meter. The process the camera uses to capture an

image stays the same whether you shoot in color mode or B&W or use Photoshop to create a

B&W.

 

BTW. I glad you like the site.

 

Thanks