Slow Lenses and Accurate Focus

I made this image using a 10X neutral density filter.  Because my Fujifilm camera is mirrorless,
I was able to compose, focus, make images and make composition, focus and exposure adjustments
with the filter attached.  With a DSLR that can't be done as the viewfinder image is black.
(click to enlarge)

Two convenient things I like about using mirrorless cameras that was largely absent in DSLRs is the ability to accurately focus in extremely low light and accurately focus slow lenses.  

I remember the first time I put a 10-stop neutral density filter on my Fuji X-T2.  As I bent forward to look through the viewfinder, I was surprised that I could see the image!  When I had done the same with any of my Nikon DSLRs, the viewfinder was black.  That made things difficult.  I had to set up the camera on a tripod, compose, set my aperture then accurately focus before I attached the ND filter.  After checking the resulting image and if it wasn't to my satisfaction, I had to take the filter off and repeat the process.  As many of you know, removing and adding filters repeatedly can result in the 'butter finger' effect where we juggle holding the filter while trying to keep our fingertips off the glass.  Sometime we aren't successful and drop the filter.  Yes, I have done just that.  Not so with mirrorless cameras.  With my mirrorless cameras, I can see the image, compose, focus, adjust and make subsequent images with never having to remove the filter at all!  Talk about technology making things convenient!

Additionally, because of the technology built into mirrorless cameras, they can focus very slow lenses quickly and accurately.  I'm talking about lenses with a maximum aperture of f /6.3, f/7.1 and even f/8 .  Throw in low light situations and the camera can still focus the lenses just fine.  It wasn't too long ago that DSLRs could not properly autofocus an f/5.6 lens.  Then the high end DSLRs improved to be able to focus those lenses and a few now can focus lenses even at f/8.  Very few, however, and very costly.  The technology, as usual, does trickle down to less expensive cameras with time so this has gotten better.

Because these mirrorless cameras can leverage technology to overcome obstacles of the past associated with fast and accurate autofocus, I think that is why we are seeing a number of lenses being introduced that, in my mind, are really slow.  For example, Canon just introduced a 100-400mm lens which has a maximum aperture of f/8 at the long end.  F/8 wide open?  I'm having a hard time with accepting that.  That would have been unheard of a couple of years ago.  Canon’s excellent and most popular mirrorless wildlife lens, the R100-500mm, has a maximum aperture of f/7.1 at 500mm.  Sony, Tamron, Sigma and Nikon all have introduced lenses for their mirrorless systems with these slower maximum apertures.  Why?  

First, mirrorless cameras, along with more sophisticated algorithms, allow the camera to boost the incoming signal (the reason you can see the image with a 10-stop ND filter) to make focusing in lower light levels much more efficient and accurate.  Second, with slower maximum apertures, lens elements and the resulting lens can be smaller and lighter weight.  They now can be close to APS-C size lenses.  (Is Fujifilm's APS-C line of excellent cameras and lenses a threat?  Probably not.)  Third, slower, smaller and lighter means less expensive and there has been a trend with photographers wanting to reduce the size, bulk and weight of their kit.  But the cost is slower maximum apertures.

All of this may be fine but I just don’t like being restricted by such slow lenses.  It has more to do with image quality than lack of shallow depth of field.  (Most of my photography requires more, not less, depth of field.)  No matter what you do, smaller apertures means higher ISOs are needed.  Higher ISOs means more signal amplification which results in more noise and less dynamic range.  Even though there are the sophisticated AI based noise reduction plug-ins, dynamic range suffers each time you turn up that ISO dial .  In many cameras when you reach ISO 6400 you've lost fully half your available dynamic range at base ISO.  Instead of having 12, 13 or even 14 stops of dynamic range, you now have 6-7 stops.  Not good.  Additionally, if you turn ISO up high enough because you may be in a low light situation and need a high shutter speed for wildlife, sports, birds-in-flight, etc. photography, a lot of resolution is lost.  The image breaks down and fine detail is lost.  Slower lenses, although convenient are not conducive to achieving the best image quality.

Although mirrorless cameras allow you to focus with smaller, lighter and slower lenses under more circumstances, there are always downsides that one has to consider.  So it is not all roses.  Always think about the other side of the coin, so to speak, when manufacturers tout technological benefits.  There are always some sort of drawbacks and it is your job to discover them before getting out your credit card.

I've set my personal target of a maximum aperture of f/5.6 as the slowest lens I would buy  so I can keep my image quality as high as possible while balancing cost, size, weight and bulk.  That said, I could be tempted by the long zooms by Sigma and Tamron which have a maximum aperture of f/6.3 at the long end.  But those lenses have only worked well for me in situations where the light is plentiful.  On heavily overcast days, the problem of high ISOs still arises.  

I hope this trend of slower lenses doesn't become the norm and faster lenses are no longer developed and manufactured.  That would serve the photo industry well as there are benefits to lower manufacturing and sales costs but not necessarily serve us photographers well.  Fingers crossed!

Join me over at my website, https://www.dennismook.com.  

Thanks for looking. Enjoy!  

Dennis A. Mook  

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