Is it just me or are there a lot of angry people here?

Started May 25, 2012 | Discussions thread
Senior MemberPosts: 1,302
Re: So, which bit
In reply to Louis_Dobson, May 28, 2012

I don't care what focal length it is - or isn't 'equivalent' to ... you CANNOT change the light transmission value by altering the format you use it on. It is - and always will be, an f1.8 lens - fixed constructional value.

I'd suggest you go read a primer on lens construction and light transmission theory - format view does not alter the f-value of a lens. It is not a f3.6 lens just because it's 'equivalent' angle of view is considered similar to a 'traditional' 150mm on a particular format.

You really do not understand what an f-stop is if you believe change in angle of view thus changes a lens's light transmission values ....

Regardless of what you think it is, or wish to call it - the camera will know better and meter for f1.8 - because that's the light value it will receive.

Louis_Dobson wrote:

Photozopia wrote:

I give up - you obviously don't get that it is not the aperture that has changed but the size of the sensor .... If it was possible to put the lens on every format in existence - subject to coverage - it would ALWAYS be a 75mm f1.8!

No, you're not following. It IS a 75 f1.8.

The problem comes when people say it is like/equivalent to/effectively a 150mm f18. It isn't. If it is equivalent to a 150 f something, then it is a 3.6.

You alter max. aperture by closing down the iris - or (theoretically) changing the physical focal length of the lens. Neither happens here - it is just that sensor size crops the angle of view to that associated with a 150mm lens.

If you crop a printed image to half size (effectively making the angle of view twice the size) you do not alter the aperture you took the original picture with - that is what the sensor is doing in this instance, simply halving the angle of view traditionally associated with a 75mm lens - it does not 'alter' it's focal length by doing so, nor restrict it's aperture. A lens will always transmit it's original focal length aperture.

The only exception is use of a lens converter/multiplier - which either PHYSICALLY increases the focal length of a lens (and/or moves it away from the image plane) by increasing the distance light travels through the original aperture mechanism - thereby altering the set f-stop value ratio.

Sensor (or film) format relationship has nothing to do with inbuilt transmission of light values (which are mathematically related in the lens construction) only in apparent, i.e. equivalent, angle of view. Equivalent to does not mean identical to!

Louis_Dobson wrote:

If we are going to call it a 75 f1.8, that's fine.

But if anyone wants to point out the "equivalent" focal length is 150mm, then it has to be pointed out the "equivalent" aperture is f3.6. The crop factor causes the effective focal length to double, and because the f number is a factor of focal length, the f number should be doubled as well to give us an effective f number (although obviously the exposure continues to match f1.8, that is what the f number is for ).

What is the lens? It is a 75mm f1.8. 75mm = 75mm, f1.8 = f1.8.

What are it's capabilities compared to an FF lens? In terms of noise, DoF and AoV, it will produce identical results to a 150 f3.6 lens with the ISO turned up two stops.

If 75mm=150mm, then f1.8=f3.6.

Photozopia wrote:

You're both arguing from the wrong perspective and as a result you are both right AND wrong.

Aperture - i.e. f stop is a mathematical relationship between focal length/physical aperture.

A 75mm f1.8 is not ( when used on a different format ) anything other than a 75mm f1.8 lens - but the crop factor of the m4/3 format gives it an effective angle of view consistent with that of a 150mm lens - that does not however offer any substance to claim it is then made a 150mm f3.6.

It is always a 75mm f1.8 - but now offers an angle of view consistent with 150mm lens. It is not the aperture that changes - it is the cropped angle of view - which is changed.

Now just shake hands and say sorry to each other for causing all this fuss .... jeez!

Louis_Dobson wrote:
of f=fl/A are you arguing with?

The lens is a 75mm f1.8. That's fine. But if you want to say it is a 150mm, you have to accept to the same basis it is an f3.6.

A 75mm f1.8 has an entry aperture of 42mm.

A 150mm f1.8 would have an entry aperture of 84mm. That's the 'ole in the front, and what most people think of when they talk of "aperture."

A 150mm with an entry aperture of 42mm is an f3.6.

DickyUK wrote:


F1.8 will let in the same amount as any other lens at f1.8

F numbers are for calculating the amount of light which is let in and allF numbers are constant F 1.8 is F1.8 IS f1.8

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