Technical Help Question for Lens Decision Making!!!

[nicolas505]

Hello everyone again,

I am still in the middle of the process of buying up my lenses... I just want to ask a very important yet technical question:

The difference between a f/3.5 and f/2.8 maximum lens in terms of actual speed in seconds would be approximately what? Considering it's the same iso speed and everything else is the same including lighting and levels of light ... so what's the actual amout of seconds gained when you have a lens opened at 3.5 and another lens opened at 2.8 in the same lighting conditions... if there's no way to be precise can you give me a rough number... thank you very much guys... i really appreciate it... and it would give me a concrete idea of what I could gain in terms of seconds in my lens decision making process... I don't know if this is the right forum, but please let me know... cheers guys xxxx

Nick

[SkipD]

You're talking about a difference of almost a full f-stop (a full f-stop up from f/2.8 is f/4.0). That would be almost twice (or half) the shutter speed to balance the difference between the two f-stops, depending on how you look at the difference.

[PetKal]

Why not try some self-help experiment here: with your camera in the Aperture priority mode change the aperture value and observe what the corresponding shutter speed values are, yes ?;)

[nicolas505]

i only have a camera that is a 3.5 max aperture... so is the rule that at every full stop you stop down, the time value doubles for the shutter speed to keep the same aperture? Or is there another formula? Thanks

[Andy_T]

Yes, that is the exact rule. Every full stop doubles the necessary shutter speed.

Full stops are multiples of 1.4 (sqare root of 2)
1.0-1.4-2.0-2.8-4.0-5.6-8.0-11.0-16.0

Best regards,
Andy

[PetKal]

i only have a camera that is a 3.5 max aperture... so is the rule that at every full stop you stop down, the time value doubles for the shutter speed to keep the same aperture? Or is there another formula? Thanks

No, that keeps the "same exposure", i.e., the integrated amount of light addmitted to the sensor thru the lens.
That exposure you can double in two ways:
*double the duration of exposure.
*Double the orifice area thru which light streams in. In case of a circle, the area doubling is achieved by increasing the diameter not twice but by sq rt of 2. Obviously, the aperture value behaves as the inverse of that diameter.

[nicolas505]

Yes, that is the exact rule. Every full stop doubles the necessary shutter speed.

Full stops are multiples of 1.4 (sqare root of 2)
1.0-1.4-2.0-2.8-4.0-5.6-8.0-11.0-16.0

Best regards,
Andy

ok great thanks so much ... is there a specific set of increments of shutter speed... just like wat u gave me for aperture?

[Jon]

The shutter speeds double. 1", 1/2", 1/4", 1/8", 1/15" (don't ask!), 1/30", 1/60", 1/125" (see 1/15"), 1/250", 1/500", 1/1000",1/2000", . . . Your finder display for speeds less than 1" will show only the denominator. For 1" and beyond, it'll show the time and a seconds (") tick.

[snibbetsj]

One other thing to consider besides shutter speed is the lens quality itself. Generally the f2.8 lens are the L zooms or the better prosumer lens. (Althought the 17-40F4 and 70-200 f4 plus the longer primes are L quality). The slower apertures, such the kit lens, have the "cheap" consumer build quality and are considered to be the "bottom of the line". These may produce decent photos but are, in general, not as sharp, shuffer from CA, lens flare, etc more so than the prosumer or L lens.

Just my $.02 worth.