[Steve Grosvenor]

When trying to find a fast lens for low light I came upon little discussion with regarding how much crop sensors influence depth of field and bokeh.

Am I correct that to match the popular DA 35/2.4, when use on an APS-C camera, would be no better than a 50/3.5 on a full frame?

If it is, then why are more photographers not looking for 'fast lenses'?

Low light performance would, with the high ISO performance of new sensors, seem to be less of a reason driving fast lenses as trying to achieve a desirable depth of field.

---------- Post added 10-29-17 at 05:37 AM ----------

The photographic world loves to talk about the 'equivalent' focal length, but equivalent aperture never seems to get mentioned.

Consider the lens on the Sony RX10. It is usually referred to as being 24-600mm / F2.4-4 Why is it not referred to as 24-600mm / F6-8?

[photoptimist]

Two answers:

1) Some photographers are looking for fast lenses (or moving to FF) to get shallow DoF.

2) Other photographers don't want shallow DoF (which just makes most of the image out of focus) and prefer avoiding the cost and weight of faster lenses.

As the high-ISO performance of cameras has improved, the need for heavy, expensive, fast lenses has dropped. It's only photographers who seek the shallow DoF look that either look for faster glass or larger sensors.

[biz-engineer]

Low light performance would, with the high ISO performance of new sensors, seem to be less of a reason driving fast lenses as trying to achieve a desirable depth of field.

It depends how low is the low light. Given acceptable DoF, and effective image stabilization, a faster lens may allow shooting without needing a tripod even in dim light.

[photoptimist]

The photographic world loves to talk about the 'equivalent' focal length, but equivalent aperture never seems to get mentioned.

Consider the lens on the Sony RX10. It is usually referred to as being 24-600mm / F2.4-4 Why is it not referred to as 24-600mm / F6-8?

The concept of "equivalent aperture" does not work when setting the exposure, managing external flash settings, or using rules of thumb such as the sunny f/16 rule.

From the standpoint of setting exposures, the lens on the RX10 really is a f/2.4-4 lens.

[aslyfox]

a large part of these articles " Hurts " my head but for someone smarter than me or who has had some coffee this AM, these might be of interest

Understanding Depth of Field in Photography


DEPTH OF FIELD REQUIREMENTS
As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).


Digital Camera Sensor Sizes: How it Influences Your Photography

Depth of Field Equivalents
Sensor #1 35 mm (full frame)

Selected Aperture f/2.0

Lens Focal Length 28 mm

Sensor #2 Digital SLR with CF of 1.5X

Required* Focal Length: 18mm

Required Aperture: F1.3

__________________

Depth of Field Equivalents
Sensor #1 50 mm (full frame)

Selected Aperture f/2.8

Lens Focal Length 33mm

Sensor #2

Digital SLR with CF of 1.5X


Required* Focal Length: 33mm

Required Aperture: f1.8

[mcgregni]

It's not the sensor size the affects the DOF. DOF is an optical effect produced by the lens. It makes no difference what sensor the lens is in front of. What is different with larger sensors is that in order to get things big enough in the frame, the focus distance has to he smaller (closer) .... It is this that results in smaller DOF at the equivalent apertures.

[pschlute]

Photographers (or should that be gear enthusiasts) are always looking for fast lenses. But what are you trying to achieve? The DA 35mm at 2.4 at a subject distance of 10 feet has a DOF range of 2.4 feet. If you put a 35mm f1.4 (much more expensive and about the limit a 35mm lens could be manufactured) you would have a DOF range of 1.4 feet. That enough of an improvement to justify the extra cost ?

[marcusBMG]

There's been a lot of discussion on this sort of stuff, often way OTT for what is a relatively small point. There are online pundits eg Tony Northrupp who really bang on about this (misleadingly IMO)...
Re the RX10 I assume that "24-600mm" is a 35mm full frame equivalent focal range. Camera manufacturers describe like this I think because they recognise that is what is most meaningful to many camera purchasers, and it allows them to have a consistent reference focal length description across different sensor sizes that will communicate the range of fields of view a camera offers.
Aperture is a bit different. Different sized sensors will capture different proportions of the image circle projected by a certain lens at a certain distance from a subject thus capturing different FoV. However the aperture doesn't change - it is a fixed property of the lens. Knowing the actual aperture is important because that relates directly to exposures, shutter speeds etc. I think that some manufacturers do refer to the equivalent aperture, that would be more consistent in a sense of going with the equivalent focal length, but if you bear in mind the real focal length eg the 35mm f2.4 you already know that you are getting more depth of field with that lens on apsc than with the 50mm on full frame while taking a pic of the same subject. So what's the issue really? Since you're using a dslr wysiwyg.

[CarlJF]

When trying to find a fast lens for low light I came upon little discussion with regarding how much crop sensors influence depth of field and bokeh.

Am I correct that to match the popular DA 35/2.4, when use on an APS-C camera, would be no better than a 50/3.5 on a full frame?

If it is, then why are more photographers not looking for 'fast lenses'?

Low light performance would, with the high ISO performance of new sensors, seem to be less of a reason driving fast lenses as trying to achieve a desirable depth of field.

---------- Post added 10-29-17 at 05:37 AM ----------

The photographic world loves to talk about the 'equivalent' focal length, but equivalent aperture never seems to get mentioned.

Consider the lens on the Sony RX10. It is usually referred to as being 24-600mm / F2.4-4 Why is it not referred to as 24-600mm / F6-8?

If you're into low light photography, the 35 2.4 will be preferable to a 50 3.5. All this equivalence thing has nothing to do with exposure. A fast lens is a fast lens, no matter the sensor size.

As for the difference in DOF, it's just good for forum discussion. In the real world, when you're shooting, if you want shallow DOF, you'll do your best to get it with the cameras and lenses you have on hand. You work with a specific goal in mind, not in relation to some theoritical equivalence relative to a format you don't use, or may even never have used. Then, why would you care about it ? Said otherwise, equivalence isn't relevant because you adapt your shooting to the gear you're using to get the results you want. You're not trying to copy what you'll do with a FF camera.

[Steve Grosvenor]

The concept of "equivalent aperture" does not work when setting the exposure, managing external flash settings, or using rules of thumb such as the sunny f/16 rule.

From the standpoint of setting exposures, the lens on the RX10 really is a f/2.4-4 lens.

Good point... except when looking at a lens, the smaller sensor requires more light.

I have no experience with the RX10, but only the HX90 with 1/2.3 sensor. Image quality degrades significantly above ISO 800. By comparison, the K70 seems not to suffer the same way until reaching ISO 12,600.

Thus, although the lens on the HX90 is actually 4.1-123mm / F3.5-6.4, to get the same 'performance' on a full frame would need a 24-680mm / F18-35. To keep things like the rule of thumb working an equivalent ISO number would need to be used.

[CarlJF]

Good point... except when looking at a lens, the smaller sensor requires more light.

But sensor noise and sensitivity have nothing to do with lens aperture. These are totally different problems. And yes, if low noise high ISO shooting is wanted, a larger sensor is better. But this is completely independent from the lens used... Notwithstanding the fact that sensor technology will play as much a role than sensor size.

[marcusBMG]

Good point... except when looking at a lens, the smaller sensor requires more light.

I think you've been "Tony Northrupp'ing". Sure, there's going to be fewer photons hitting a smaller sensor, and smaller photocell sites, in total (= grainier pictures at higher ISO due to higher Signal/Noise ratios). But its light intensity that detemines exposure, and that is independent of the sensor or photocell sizes.
So if my sense of what you mean when you say " the smaller sensor requires more light" is correct, well no it doesn't

[BrianR]

Am I correct that to match the popular DA 35/2.4, when use on an APS-C camera, would be no better than a 50/3.5 on a full frame?

As far as Depth of Field and background blur when used wide open, they would be pretty much the same (on paper anyway, real life examples may render things pretty differently).

Weight and size are not huge for your example lens, but compare it with the FA 35/2. It's over twice as expensive as the DA 35/2.4, and that's not even a full stop of aperture you're getting. Then compare these to the sigma 35/1.4 art, it's monstrous in size and price. It's always a compromise, and with an inexpensive lens like the DA 35/2.4, you don't get to complain about a stop and a half when you have enough money left over to buy 3 or 4 spares.

Consider the lens on the Sony RX10. It is usually referred to as being 24-600mm / F2.4-4 Why is it not referred to as 24-600mm / F6-8?

In addition to other problems mentioned above, it is less impressive from a marketing standpoint. This is assuming the telephoto hungry consumer knows enough that a lower f-number means more light. "I have a 600mm f/4 lens that I can fit in my handbag." Great for camera salespeople who like to ignore physics.

[Steve Grosvenor]

However the aperture doesn't change - it is a fixed property of the lens.

But focal length is also a fixed property of a lens, yet to provide more 'useful' information for an enthusiast the norm is to state 'equivalent focal length'.

If I have a FF with a 50mm/F2 lens I have an idea what I will be able to achieve with regards to focal depth, light sensitivity and width of view.

Back in the day of film there really was only one film size for the majority - 35mm. Problem now is that there are 4 competing sensor sizes for the masses - 1", 4/3, APS-C and 35mm.

Now instead of being able to judge the 'performance' of a camera simply by looking at the focal length and max aperture, one has to consider two additional numbers (sensor size and max usable ISO).

An adjusted aperature simply makes it easier (like adjusted focal length) to compare apples to oranges.

---------- Post added 10-29-17 at 07:23 AM ----------

... it is less impressive from a marketing standpoint...

That sums it up nicely... it has everything to do with marketing, and very little to do with providing useful information so the average joe can make an informed decision.

---------- Post added 10-29-17 at 07:23 AM ----------

... it is less impressive from a marketing standpoint...

That sums it up nicely... it has everything to do with marketing, and very little to do with providing useful information so the average joe can make an informed decision.

[pschlute]

But focal length is also a fixed property of a lens, yet to provide more 'useful' information for an enthusiast the norm is to state 'equivalent focal length'.

I think "equivalent focal length" is the single most confusing thing that digital camera manufacturers came up with when they started marketing the aps-c sized sensor.

I have been snapping away happily for 40 years. i never felt the need to know what my 35mm film lens was in eqivalence with a 6x7 or 645 camera. With my Pentax DS and K10D I cannot recall ever thinking that the DOF was significantly different from my Z-1

I use a k1 now. If I want less DOF I open the lens up more, or move closer to the subject, or slap a faster lens on the camera.