Prime vs. Zoom speed: I have a question concerning a difference I have noticed since I have been using Prime lenses more often. When using a Zoom lens and compare the settings with a...

CLF Loc: Raleigh, NC

I have a question concerning a difference I have noticed since I have been using Prime lenses more often.

When using a Zoom lens and compare the settings with a Prime lens of the same length (ie a 100-400 L set to 300mm vs a 300mm L) it seems the Prime lens lets in at least one/two stops faster than the Zoom lens.

The question:
Does a Prime lens allow more light to pass through than a Zoom?

Thank you in advance for your comments.

Greg

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I just compared the amount of elements between the Canon 100-400 L and the 300mm L and the zoom has 2x the amount of elements.

Greg

Dik

Yes. f/stops are a dimensional calculation. T-stops are accurate light transmission values.

CLF Loc: Raleigh, NC

Dik wrote:

Yes. f/stops are a dimensional calculation. T-stops are accurate light transmission values.

Thank you, Dik. I appreciate your comment and will compare the two.

Greg

wdross Loc: Castle Rock, Colorado

CLF wrote:

I have a question concerning a difference I have noticed since I have been using Prime lenses more often.

When using a Zoom lens and compare the settings with a Prime lens of the same length (ie a 100-400 L set to 300mm vs a 300mm L) it seems the Prime lens lets in at least one/two stops faster than the Zoom lens.

The question:
Does a Prime lens allow more light to pass through than a Zoom?

Thank you in advance for your comments.

Greg

In general, primes are easier to be designed faster than zooms. For a zoom to be as fast as some of the fastest primes would require for the zoom to be many times bigger and heavier. Any lens listed at a specific f-stop has the same amount of light coming through it as any other lens at the same specific f-stop.

CLF Loc: Raleigh, NC

wdross wrote:

In general, primes are easier to be designed faster than zooms. For a zoom to be as fast as some of the fastest primes would require for the zoom to be many times bigger and heavier. Any lens listed at a specific f-stop has the same amount of light coming through it as any other lens at the same specific f-stop.

Thank you, Wdross.

Greg

Dik

Wrong wdross,
T-stop lenses transmit the same amount of light at the same setting, but f/stop lenses do not.
I confirmed this myself shooting a Canon 24-105L at 40mm f/4, and a Voightlander Ultron 40mm f/2 at f/4.
There was a little more than a stop difference. I had to set the prime to f/5.6+ to match the zoom .

wdross Loc: Castle Rock, Colorado

Dik wrote:

Wrong wdross,
T-stop lenses transmit the same amount of light at the same setting, but f/stop lenses do not.
I confirmed this myself shooting a Canon 24-105L at 40mm f/4, and a Voightlander Ultron 40mm f/2 at f/4.
There was a little more than a stop difference. I had to set the prime to f/5.6+ to match the zoom .

I have set my primes and zooms using a light meter to the f-stop indicator by the meter and have gotten identical exposures. I have also shot with a prime and changed to a zoom and used the prime's exposure to get the same exposure with the zoom. There may be something wrong with the mechanics of one of your lenses.

Dik

I believe lens makers use f/numbers because they are needed for depth of field calculations. Cine lenses are marked with T-stops, for precise exposure settings using external metering.

Dik

wdross -
No, do some research on T-stops.
Repeat your experiment, and read the RGB values in Photoshop.
f/numbers do not take into account the light that is absorbed or scattered by each element of the lens.
So zoom lenses with lots of elements absorb more light than primes with fewer.
T-stops are measurements of the light transmission of the lens, f/stops are derived from dimensions and do not reflect the actual amount of light transmitted.

kymarto Loc: Portland OR and Milan Italy

Dik wrote:

Wrong wdross,
T-stop lenses transmit the same amount of light at the same setting, but f/stop lenses do not.
I confirmed this myself shooting a Canon 24-105L at 40mm f/4, and a Voightlander Ultron 40mm f/2 at f/4.
There was a little more than a stop difference. I had to set the prime to f/5.6+ to match the zoom .

F stops are a mathematical measure of the ratio between focal length and lens diameter. T stops take into account transmission loss through the glass. Still a one stop difference between two lenses seems excessive.

wdross Loc: Castle Rock, Colorado

Dik wrote:

wdross -
No, do some research on T-stops.
Repeat your experiment, and read the RGB values in Photoshop.
f/numbers do not take into account the light that is absorbed or scattered by each element of the lens.
So zoom lenses with lots of elements absorb more light than primes with fewer.
T-stops are measurements of the light transmission of the lens, f/stops are derived from dimensions and do not reflect the actual amount of light transmitted.

I understand what you are indicating. But based off my meter readings and getting identical exposures from either my primes or zooms off those those reading, Olympus is putting on their lenses the actual transmitted light as the f-stop. Otherwise one of the lenses would deviate from the meter reading and give an incorrect exposure.

Dik

wdross wrote:

I understand what you are indicating. But based off my meter readings and getting identical exposures from either my primes or zooms off those those reading, Olympus is putting on their lenses the actual transmitted light as the f-stop. Otherwise one of the lenses would deviate from the meter reading and give an incorrect exposure.

wdross-
I think you are being fooled by in-camera metering.
Set everything to manual, run a test shooting a graycard with a complex zoom and a simple prime, bracket 1/2 stop intervals for 2 stops and check which images match.

Dik

If the lens markings are transmitted light values they are marked T, if they are labeled f/xx, or F:xx, they are not transmitted light values.

jerryc41 Loc: Catskill Mts of NY

CLF wrote:

I have a question concerning a difference I have n... (show quote)

Wouldn't that depend on the F/ specifications of each lens? Primes are usually faster than zooms. Pay the big bucks, and you can get an F/2.8 zoom, but most are F/3.4 - F/6.3, or something like that. Zooming in will decrease the aperture.

Dik

f/, F/, F: all refer to the focal length divided by the diameter of the aperture. They are dimensional measurements.

jerryc41 wrote:

Wouldn't that depend on the F/ specifications of each lens? Primes are usually faster than zooms. Pay the big bucks, and you can get an F/2.8 zoom, but most are F/3.4 - F/6.3, or something like that. Zooming in will decrease the aperture.