Big Tall And Small

What Big Tall And Small Actually Means In Practice

Big Tall And Small is a multi-scale imaging strategy used mostly in astrophotography and landscape photography. The idea is straightforward: you use different equipment and exposure approaches for different subject sizes within the same frame, then stack or blend them together. You don't shoot everything with the same lens at the same settings and hope it works out. I learned this the hard way. A few years ago I was trying to photograph a Milky Way core rising behind a mountain ridge. I used a 14mm f/1.4 on a full-frame camera, shot 25 seconds at ISO 3200, and ended up with a foreground that was nicely exposed but a sky that was blown out in the brightest regions. The alternative—longer exposures for the sky—would have created star trails. Shorter exposures would have left the foreground in the mud. That's exactly the problem BTAS solves. The method involves taking separate exposures optimized for each scale of subject. The "big" part refers to wide-field or large-format captures that get the overall scene. The "tall" part is usually a longer focal length or telephoto pass targeting specific elevated or extended features. The "small" part is your short-exposure or higher-resolution pass for fine detail work. You combine them in post-processing.

Setting Up Big Tall And Small For Astrophotography

Here is how I actually do it when the weather cooperates and the light pollution isn't absolutely terrible. First, I mount the camera on a tracked equatorial mount for the deep-sky portions. I shoot my narrowband or LRGB data separately from my wide-field Ha/RGB data. The key insight most beginners miss is that you don't need to physically switch lenses between passes if your sensor resolution and field of view allow it. I sometimes use a single 72mm f/4 refractor for both the wide and tall passes by cropping the center portion in post. It costs you some field of view on the wide pass, but it saves you a lot of time on re-framing and recalibration. For the actual workflow, I start with the tallest focal length first because the framing is most critical and the setup is most stable when the scope is still cold. I collect my narrowband data—OIII, SII, H-alpha—typically 60 to 90 sub-exposures depending on sky conditions and whether I'm imaging in Bortle 4 or worse. Then I switch to the wide-field setup, usually a 24mm f/1.4 or similar, and grab my RGB and star field data. The transition takes maybe ten minutes if my setup is decently engineered.

The blending happens in Photoshop or PixInsight. I stack the narrowband data, apply PCA or LSDR for colorization, then composite it over the wide-field RGB star field. The result is a image where the nebulae are rich in detail and color, but the surrounding galaxy field and foreground remain natural and properly exposed. One edge case I ran into that almost ruined a project: I once shot my wide-field pass on a night with high cirrus clouds that weren't visible to the naked eye. The H-alpha channel picked up thin cloud structure that was completely invisible in the RGB channels. When I blended the narrowband data on top, the clouds created ghostly artifacts in the final image. I caught it only because I was doing a test export at full resolution. The workaround was to create a mask from the H-alpha light frames alone, identify the cloud-affected frames, and exclude them before stacking. Takes about five extra minutes and saves you from wasting three hours on bad data.

Common Mistakes People Make With BTAS

The biggest issue I see is that people treat this as a single-shot technique. It isn't. If you try to capture everything in one exposure triangle, you will compromise on all of it. The dynamic range between a nebula core and the surrounding star field is often 6 to 8 stops. No single camera sensor handles that gracefully without significant tone-mapping artifacts. Another mistake is not maintaining consistent calibration frames between passes. Flat fields and dark frames are pass-specific. The temperature difference between your narrowband setup and your wide-field setup can shift the thermal noise pattern enough that reusing calibration frames introduces artifacts. I take new darks and flats for each rig configuration. It adds maybe twenty minutes to the session but keeps the data clean. The technique also has real limitations. It does not work well when you cannot return to the same location between passes—that is, if the target moves out of frame or the lighting changes significantly. It requires either a tracking mount that stays aligned between equipment changes or a rigid pier setup that holds alignment. For casual photographers without a goto mount, the practical benefit diminishes quickly because the time spent re-aligning and re-framing eats into useful exposure time.

If you are working in extremely bright light-polluted areas where narrowband loses most of its advantage, the BTAS approach becomes less worthwhile. The signal-to-noise ratio on the nebula data simply does not improve enough over a well-processed RGB-only image to justify the extra complexity. In those cases, a single well-executed wide-field RGB pass with good noise reduction tools is often the better use of your time. For those interested in learning more or getting started with the equipment commonly used in this workflow, there are several community resources and software guides available online. The core principle remains the same regardless of your specific gear: match your exposure strategy to your subject scale, not the other way around.